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LTIVRARY 

UNIVERSITY  OF  CALIBOKN* 
PAVIS 


DUPLICATE 


MEMOIRS 

PRESENTED   TO   THE   CALIFORNIA   ACADEMY   OF   SCIENCES. 


VOLUME    I. 


II.     Principles  of  the  Natural  System  of  Volcanic  Hocks. 
BY  F.  BARON  RICHTHOFEN,  DR.  Fun.. 

[Presented,  May  6th,  1867.] 

INTRODUCTORY.  Among  the  features  peculiar  to  modern  Geology  may  be  noticed 
a  revival  of  that  speculative  tendency  which  prevailed  among  the  cultivators  of  this 
science  at  the  close  of  the  last  century.  But  while  in  those  early  times  imagination 
exerted  a  dominant  influence  in  the  framing  of  hypotheses,  and  discussions  between 
the  adherents  of  different  doctrines  were  conducted  with  all  the  bitterness  peculiar  to 
such  struggles,  when  neither  party  has  a  firm  basis  upon  which  to  found  its  arguments, 
the  constant  ascendency  of  the  spirit  of  the  inductive  method  has  imparted  to  those 
theories  more  recently  propounded  a  more  logical  and  scientific  form,  while,  at  the 
same  time,  the  increasing  amount  of  positive  knowledge  has  given  to  the  different 
doctrines  a  more  varied  and  more  definite  character,  and  enlarged  the  scope  of  dis- 
senting views. 

This  renewed  tendency  to  systematize  and  theorize,  which  is  especially  con- 
spicuous in  the  records  of  the  last  twenty  years,  must  be  ascribed,  partly,  to  the  vast 
amount  of  well-established  facts  gathered  during  the  previous  decades,  and  which 
have  since  been  multiplied  and  intensified  in  a  constantly  increasing  ratio,  as  regards 
depth  and  distinctness  of  observation  as  well  as  the  geographical  area  over  which  they 
extend  ;  partly,  and  in  no  less  degree,  to  the  rapid  progress  made  by  those  sciences  on 
which  geology  has  to  draw  for  the  general  laws  which  are  alone  capable  of  affording  a 
philosophical  guide,  to  speculation  on  the  basis  of  facts  gained  by  observing  and  com- 
paring. The  advance  of  the  chemical  and  physical  sciences,  especially,  has  had  a 

MEM.    CAL.   ACAD.   SCI.    VOL.    I.  [2u]  Jan.    1868.  (39) 


2  INTRODUCTOEY. 

powerful  influence,  by  allowing  an  immediate  application  to  geological  problems  of 
such  general  laws  as  are  established  beyond  any  doubt.  Besides,  the  improvement  of 
the  practical  methods  applied  in  chemical  and  physical  laboratories  has  given  rise  to 
the  execution  of  numerous  experiments,  which  were  made  with  a  view  of  imitating  the 
processes  applied  in  the  great  natural  laboratories.  They  suggested  frequently  the 
causes  of  facts  disclosed  by  the  examination  of  a  certain  region,  or  obtained  by  com- 
paring the  observations  made  in  different  countries  on  one  certain  subject. 

It  appears  to  be  mainly  due  to  these  causes  that  a  number  of  theories  have 
been  proposed,  in  rapid  succession,  relating  to  the  origin  of  rocks,  to  the  mode  and  causes 
of  metamorphism,  to  the  agencies  of  vulcanism,  to  the  structure  and  mode  of  formation 
of  mountain  ranges,  to  the  structure  of  the  entire  globe,  and  other  cognate  subjects. 
It  will  be  admitted,  even  by  those  who  are  most  strongly  opposed  to  theorizing,  that 
geological  science  has  in  this  way  been  promoted  and  enriched  in  various  respects  ; 
since  there  is  scarcely  a  theory  which,  even  if  insufficient  to  explain  what  it  under- 
takes, has  not  some  truth  in  it,  or  is  applicable  to  some  extent  in  certain  cases,  or  has 
at  least,  even  when  proving  to  be  erroneous  itself,  led  to  discussions  on  subjects  of  high 
interest,  which  is  indeed  a  result  of  no  little  value.  In  reviewing  the  theories  pro- 
posed on  any  particular  subject,  we  find  them,  it  is  true,  often  in  apparent  contradic- 
tion with  each  other  ;  yet  almost  every  one  is  based  upon  arguments  drawn  from 
observed  facts,  and  there  are  probably  very  few  which  will  ultimately  be  entirely 
abandoned.  The  immense  range  of  varied  processes  as  applied  by  nature  allows  the 
applicability,  in  a  limited  way,  of  many  a  theory  in  certain  instances,  while  in  others 
it  may  be  refuted  on  no  less  valid  grounds  ;  and  the  struggle  between  the  defenders  of 
different  doctrines  is  often  founded  only  in  the  difference  of  their  standing-points. 
What  appears  to  be  true  in  one  instance  is  frequently  not  applicable  in  others  ;  it  is 
the  bold  generalizations  which  render  theories  so  often  untenable  in  that  form  in  which 
they  are  usually  first  expressed.  An  instructive  example  is  presented  by  the  different 
theories  which  have  been  proposed  for  explaining  the  mode  of  formation  of  mineral 
veins.  Almost  every  one  of  them  was  based  upon  a  limited  range  of  observations,  and 
was,  from  its  first  application  to  a  few  instances,  extended  to  the  generality  of  veins. 
Numerous  exceptions  to  it  were  then  found,  leading  to  the  rejection  of  the  first,  and 
the  establishment  of  a  different,  theory,  which,  in  its  turn,  shared  a  similar  fate.  Ob- 
scure as  this  subject  still  is,  we  are  able  to  state  this  as  certain,  with  our  present  state 
of  knowledge,  that  every  mineral  vein  is  the  product,  not  of  one  simple  but  of  complex 
processes.  Nearly  every  one  of  the  theories  proposed  will,  therefore,  have  its  limited 
range  of  applicability,  inasmuch  as  the  agent  it  suggests  may  have  been  especially 
active  in  the  formation  of  the  veins  of  a  certain  order,  while  the  same  agent  may  have 
played  a  subordinate  part  in  regard  to  the  origin  of  other  veins  which  were  chiefly 
due  to  processes  of  another  kind. 

This  instance  points  clearly  towards  the  one  principal  cause  of  the  divergence 
of  opinions  in  regard  to  some  of  the  most  important  geological  questions.  This  cause 
is  the  want  of  latitude  of  the  basis  upon  which  arguments  are  founded.  Conclusions 
which  are  obtained  by  reasoning  on  geological  subjects  solely  on  the  strength  of  chem- 
ical analysis,  are,  when  generalized,  often  found  to  be  utterly  in  discordance  with  the 
( 1") 


INTRODUCTORY.  6 

facts  revealed  by  geological  observation  ;  and  how  unsatisfactory  general  theories  may 
be  when  based  upon  the  latter  alone,   is  sufficiently  exemplified  by  the  fantastical 
attempts  made  in  all  ages  of  geological  science  to  interpret  the  geological  structure  of 
the  world  from  that  of  a  limited  region.     The  basis  for  argumentation  can  therefore 
never  be  broad  enough,  and  its  enlargement  should  be,  as  it  indeed  is,  one  of  the 
chief  objects  of  geological  science.     But  it  is  not  sufficient  to  content  ourselves  with 
an  accumulation  of  primary  observations,  which  are  in  fact  being  infinitely  increased 
by  the  conjoint  labors  of  geologists  in  all  countries  :  it  should  be  a  higher  object  of  the 
student  of  geology,  to  compare  the  established  results  of  observation,  and  to  investi- 
gate their  mutual  relations.     The  study  of  the  structure  of  one  mountain  range,  or  of 
several  ranges  comprised  within  a  limited  district,  may  lead  to  the  establishment  of  an 
elaborate  theory  of  the  mode  of  their  formation,  which  may  apparently  answer  per- 
fectly well  in  that  one  case,  but  may  be  found  inadmissible  when  generalized,  even  in 
those  cases  where,  by  imperfect  observation,  one  would  expect  to  detect  a  great  simi- 
larity to  the  structure  first  observed.     But  in  determining  those  features  which  are 
common  to  a  number  of  mountain  ranges,  or  to  certain  orders  of  them  which  we  may 
discern  among  their  generality,  we  may  aspire  to  form  conclusions  which  are  more 
generally  applicable.     It  is  particularly  the  auxiliary  branches  of  geology  to  which 
these  remarks  apply.     The  value  of  observations  made  in  limited  regions,  or  from  a 
limited  point  of  view,  on   subjects  such  as  the  outlines  of  the  morphological  feat- 
ures of  the  continents,  the  occurrence  of  mineral  springs,  the  structure  of  mineral 
veins,  the  age  of  those  among  them  which  carry  a  certain  metal,  the  generality  of  vol- 
canic phenomena,  the  mode  of  action  of  earthquakes,  the  nature  of  certain  kinds  of 
rocks,  and  their  part  in  the  structure  of  the  surface  of  the  globe — cannot  be  fully  real- 
ized unless  the  comparative  method  is  applied  in  as  wide  a  scope  as  we  may  be  able  to 
do,  and  the  mutual  relations  among  the  different  modes  of  manifestation  of  force,  or 
among  the  properties  of  the  kinds  of  matter  upon  which  it  acts,  or  the  bearing  of  all 
these  relations  to  each  other  and  to  the  evolution  of  the  globe,  are  investigated  from 
as  many  points  of  view  as  we  may  detect,  and  in  as  many  combinations  as  possible. 
We  may  then  be  able  to  gain  a  foundation  for  argumentation  on  more  involved  prob- 
lems, consisting  not  of  imperfect  premises,  nor  of  a  confused  accumulation  of  facts, 
but  of  established  truths  of  a  higher  order. 

The  mode  of  origin  of  the  non-foliated  crystalline  rocks,  made  up  of  silicates,* 
is  among  those  subjects  which  have  at  all  times,  but  at  no  time  more  than  of  late, 
commanded  a  great  deal  of  attention,  and  given  rise  to  the  establishment  of  numerous 
theories,  each  of  which  was  applied  in  a  general  way,  if  not  by  its  author,  then  by  his 
followers.  It  is  well  known  how  conflicting  they  apparently  are,  and  what  weighty  ar- 
guments have  been  brought  in  favor  of  as  well  as  against  each  of  them.  The  only 
method,  promising  success,  of  weighing  the  merits  of  these  different  theories,  or  of 
modifying  them  in  accordance  with  the  general  advance  of  science,  appears  to  be,  to 

*  I  have  for  these  applied  the  name  "  eruptive  rocks "  in  the  following  pages,  considering  that,  wherever  we  have 
occasion  to  observe  them,  they  are  not  at  their  original  seat,  but  ejected  from  it  towards  the  surface.  The  reasons  supporting 
this  position  will  be  more  fully  mentioned  in  the  chapter  on  the  origin  of  volcanic  rocks. 

(41) 


4  INTRODUCTORY. 

ascend  from  the  examination  of  the  nature  of  these  rocks  to  that  of  their  mutual 
relations,  to  investigate  these  from  as  many  points  of  view  as  we  can  discover,  in  re- 
gard to  physical  and  chemical  properties,  mode  of  occurrence  and  age,  as  well  as  in 
regard  to  geographical  distribution  :  that  is,  to  try  to  establish  the  natural  system  of 
eruptive  rocks.  The  results  so  obtained  may  then,  in  accordance  with  what  we  just 
remarked  in  a  general  way,  be  applicable  to  reasoning  on  remoter  questions,  of 
which  we  can  only  attempt  to  find  the  most  probable  solution.  They  regard  chiefly 
the  causes  of  those  relations,  the  mode  of  origin  of  the  eruptive  rocks,  and  the 
processes  connected  with  their  ejection.  The  intricate  nature  of  the  subject,  and 
the  fact  that  the  present  changes  on  and  below  the  face  of  the  globe,  as  well  as 
the  events  of  the  past,  are  often  but  dimly  and  imperfectly  perceptible  to  our  ob- 
servation, demand  that  we  should  concentrate  our  endeavors  in  exploring  first  the 
laws  of  that  which  is  definite  and  constant  within  the  infinite  range  of  phenomena,  and 
await  further  experience  to  arrive  at  an  explanation  of  those  isolated  facts  which  form 
apparent  exceptions  to  the  order  of  things. 

It  is  with  these  views  that  the  following  pages  were  written.  They  extend 
chiefly  over  the  comparatively  limited,  and  yet  very  extensive  class  of  "volcanic  rocks," 
and  are  offered  as  a  mere  elementary  attempt,  which  is  necessarily  very  imperfect. 
The  application  to  exact  reasoning  of  the  numerous  observations  which  have  been  made 
on  the  subject  of  volcanic  rocks  in  different  countries,  is  nearly  prevented  by  the  ex- 
traordinary discrepancy  existing  in  regard  to  the  mode  in  which  the  names  of  rocks 
are  used  by  different  authors.  The  first  condition  of  a  uniform  and  harmonious  mode 
of  observation  on  volcanic  rocks,  and  the  phenomena  connected  with  them,  is  the  ap- 
plication of  a  uniform  system  of  nomenclature. 

In  concluding  these  preliminary  remarks,  I  dare  express  the  hope  that  some  in- 
dulgence may  be  had  with  the  imperfections  of  this  essay,  if  it  is  taken  into  considera- 
tion that  it  was  written  on  the  Pacific  coast,  where  chemical  laboratories  are  unknown, 
libraries  scarce,  and  little  opportunity  is  afforded  of  becoming  acquainted  with  the 
current  geological  literature.  I  fulfill  a  deep-felt  duty  if  I  tender  at  this  place  my 
sincere  thanks  to  Professor  J.  D.  Whitney,  not  only  for  allowing  me  the  use  of  his 
library  and  revising  the  manuscript  of  this  essay,  but  also  for  the  interest  which  he  has 
constantly  taken  in  my  pursuits,  and  for  what  I  owe  to  his  personal  intercourse,  especially 
in  a  country  where  scientific  communication  is  so  extremely  limited.  The  influence 
of  this  intercourse  will,  long  after  this,  be  kept  in  grateful  memory  by  all  those  who  are 
taking  personally  a  part  in  the  development  of  the  California  Academy,  the  members 
of  which  kindly  allowed  this  paper  to  be  published  in  their  Memoirs. 


(42) 


THE  NATURAL  SYSTEM  OF  VOLCANIC  ROCKS. 


In  reviewing  the  various  attempts  which  have  been  made  towards  a  classifica- 
tion of  eruptive  rocks — that  is,  those  crystalline  rocks  made  up  of  silicates,  which, 
without  showing  themselves  any  traces  either  of  stratified  deposition  or  foliation,  enter 
into  the  structure  of  the  surface  of  the  globe  in  such  a  way  as  to  be  unconformable 
with  the  stratification  of  the  neighboring  sedimentary  rocks,  and  as  a  rule  to  abut 
against  them  without  any  gradual  passage — we  are  struck  by  the  observation  that,  if 
they  are  based  on  any  principles  at  all,  these  are  usually  artificial,  while  none  but  un- 
satisfactory results  have  been  obtained  when  the  application  of  natural  principles  has 
been  tried.  This  want  of  success  is  the  more  striking  if  we  consider  that  it  is  peculiar 
to  petrology,  and  that  the  efforts  made  in  the  same  direction  with  other  branches  of 
descriptive  natural  sciences  have  been  attended  by  extraordinary  results.  In  zoology 
and  botany,  the  natural  system  has  long  since  been  considered  as  the  ultimate  object 
of  scientific  research  ;  and  since  the  time  when  its  first  outlines  were  discovered,  the 
progress  of  these  sciences  has  been  admirable.  Since  then  only  have  the  developments 
of  their  different  branches  cooperated  harmoniously  towards  one  common  end  :  the 
profoundest  investigations  into  the  anatomy  of  animals  and  plants,  the  study  of  their 
geographical  distribution  in  modern  time,  and  of  their  gradual  development  in  past 
ages,  have  in  their  final  results  but  been  subservient  to  the  establishment  of  a  founda- 
tion of  the  natural  system,  and  the  ingenious  deductions  made  by  Mr.  Darwin  on  the 
origin  of  species  are  but  its  philosophical  interpretation.  As  regards  mineralogy,  clas- 
sification was  for  a  long  time  a  simple  enumeration  of  minerals,  governed  by  cer- 
tain artificial  principles.  A  new  era  was  inaugurated  for  this  science  by  the  progress 
of  chemistry,  and  its  application  to  mineralogy,  by  Berzelius.  It  led  to  a  more  correct 
estimate  of  those  principles  which  had  been  formerly  applied,  and  to  the  discovery  of 
the  existence  of  an  intimate  connection  between  crystallographical  form  and  chemical 
composition.  The  combination  of  these  two  principles  gave  rise  to  the  natural  system 
of  minerals,  which  since  their  adoption  has  been  constantly  gaining  in  completeness. 

These  are  results  which  surpass  in  a  surprising  degree  those  obtained  in  regard 
to  the  natural  classification  of  eruptive  rocks.  Even  the  most  recent  and  elaborate  sys- 
tematical arrangements,  as  those  proposed  by  C.  F.  Naumann,  P.  Senfft,  B.  v.  Cotta, 
and  J.  Roth,  though  marking  a  conspicuous  progress,  are  based  on  almost  purely  arti- 
ficial principles.  In  no  other  branch  of  the  descriptive  natural  sciences,  it  is  true,  do 
difficulties  arise  so  great  as  those  which  present  themselves  in  petrology.  Prominent 

MEM.  CAL.  ACAD.  sci.  VOL.  I.  F  Jan.  1868.  (43) 


6  RICIITHOFEN  —  THE    NATUR         SYSTEM 

among  these  is  the  entire  absence  of  what  we  could  call  '•  genus,"  or  "  species,"  not  to 
mention  "  individual."  If  we  should  succeed  iu  discovering  some  natural  group  to 
which  we  might  apply  the  term  "family,"  (though  even  this  can  never  be  used  in 
petrology  in  as  definite  a  sense  as  it  is  in  the  organic  kingdoms)  we  should  find 
it  made  up  of  an  infinite  number  of  varieties  ;  and  if  we  should  be  able  to 
establish  several  groups,  the  main  types  of  which  are  conspicuously  distinct  in 
nature,  we  should  find  them  linked  together  by  gradual  passage  in  chemical  and  min- 
eral composition.  It  appears  indeed  utterly  impossible  to  draw  distinct  boundaries 
between  cognate  groups.  Certain  names,  such  as  those  of  granite,  syenite,  quartzose 
porphyry,  trachyte,  basalt  and  others,  have  been  applied  to  designate  distinct  types  of 
crystalline  rocks,  which  can  easily  be  recognized  wherever  met  with.  But,  practically, 
they  have  to  be  used  for  larger  groups  of  rocks,  in  which  those  distinct  types  appear 
like  luminous  centers,  surrounded  by  clouds  of  varieties  blending  with  each  other  in 
such  a  way  as  often  to  render  it  arbitrary  whether  to  bring  a  certain  rock  within  one 
or  the  other  denomination.  But  there  are  other  groups  of  rocks,  of  larger  dimensions 
than  the  former,  the  nomenclature  of  which  is  far  more  indistinct,  and  which,  in  regard 
to  classification,  may  indeed  be  said  to  be  still  in  an  entirely  nebulous  state.  The 
vague  and  arbitrary  mode  in  which  different  names  are  used  for  them  shows  plainly 
that  difficulties  in  regard  to  them  are  greater  than  with  other  rocks.  As  will  be  seen 
in  the  sequel,  this  indistinctness  of  external  character,  as  well  as  of  designation,  applies 
particularly  (with  the  exception  of  basalt)  to  those  rocks  in  the  composition  of  which 
silica  takes  a  less  prominent  part,  while  those  which  are  richer  in  silica  offer  much 
more  distinct  characters.  Most  conspicuous  among  the  names  applied  for  the  former 
are  those  of  "  trap,"  "  greenstone,"  and  "  porphyry."  The  latter  two  may  be  con- 
veniently used  to  designate  groups  of  rocks  having  certain  external  characters  in  com- 
mon, but  as  generic  terms  they  should  all  be  completely  abolished.1  They  never  con- 
vey a  definite  conception,  as  each  of  them  is  used  for  a  great  variety  of  rocks,  and 
they  have  only  too  often  been  made  to  serve  as  a  convenient  cloak  to  cover  ignorance. 
The  perception  of  these  difficulties  has  caused  the  idea  to  be  almost  universally 
accepted,  that  only  an  artificial  system  of  eruptive  rocks  can  be  established ;  that  is, 
that  classification  should  be  made  dependent  on  one  certain  principle  previously  as- 

1  This  applies  chiefly  to  the  term  "  Trap,"  (or  trapp)  which  had  originally  a  definite  meaning,  but  has  gradually  been 
extended  with  wonderful  elasticity.  It  was  first  introduced  by  Torbcrn  Bergmann  for  a  very  ancient,  dark  colored,  augitic 
rock  of  Uddevalla,  in  Sweden,  which  is  arranged  in  superposed  layers  abutting  against  the  slope  of  the  hill  in  the  shape  of  a 
stairway  (trappar  in  Swedish).  This  rock  would  be  called  "diabase"  in  modern  nomenclature.  The  name  "trapp"  was 
then  applied  to  other,  and  gradually  to  all,  dark  colored  eruptive  rocks,  particularly  to  such  as  were  found  occurring  in 
dykes ;  afterwards  the  "  greenstones"  (which  name,  too,  has  shown  itself  capable  of  wonderful  distension)  were'included  in 
that  denomination ;  and  finally  it  has  become  still  more  comprehensive,  though  not  quite  to  the  same  extent  with  every  au- 
thor. Its  present  meaning  may  best  be  seen  from  the  following  passage  by  Lyell,  (Elements  of  Geology,  6th  Ed.,  1866,  p. 
601  of  Am.  Ed.) :  "  This  term  (lava)  belongs  more  properly  to  that  (melted  matter)  which  has  flowed  either  in  the  open  air 
or  on  the  bed  of  a  lake  or  sea.  If  the  same  fluid  has  not  reached  the  surface,  but  has  been  merely  injected  into  fissures 
below  ground,  it  is  called  trap."  Thus,  the  same  name  which  was  originally  applied  to  a  distinct  rock  of  ancient  origin  has 
baen  generalized  so  as  to  express  now  a  mode  of  occurrence,  and,  what  is  more  remarkable,  a  mode  of  occurrence  the  very 
reverse  of  that  exhibited  by  the  original  type,  which  must  be  supposed  to  have  been  flowing  on  the  bed  of  the  sea,  and  the 
position  of  which  in  relation  to  neighboring  rocks  can  never  be  explained  by  assuming  it  to  have  been  "  injected  into  fissures 
below  ground."  Would  it  not  be  better  to  drop  such  a  name  altogether  ? 

(44) 


OF   VOLCANIC    ROCKS.  I 

sumed  as  the  point  of  issue.  Taking  crystalline  texture,  lack  of  stratification  as  well 
as  of  foliation,  and  the  fact  of  their  being  made  up  of  silicates  to  be  the  characteristic 
features  of  eruptive  rocks  iu  general,  the  most  obvious  external  differences  among 
them  are  caused  by  the  variations  of  texture  and  color.  During  the  early  stages  of 
petrographical  science,  rocks  were  therefore  classified  on  these  principles.  The  terms 
trap,  porphyry,  pearlstone,  obsidian,  lava,  amygdaloid,  wacke,  as  well  as  green- 
stone, black  porphyry,  green  porphyry,  and  others,  are  the  remnants,  in  our  present 
nomenclature,  of  that  epoch  when  the  more  minute  differences  of  rocks  arising  from 
their  mineral  composition  were  but  imperfectly  investigated.  This  principle  was 
necessarily  next  in  order  for  serving  as  point  of  issue  for  classification,  since,  as  far  as 
regards  external  characters,  it  is  only  second  to  the  former  in  value.  The  emer- 
gence of  petrology  from  a  chaotic  state,  by  the  scientific  application  of  this  prin- 
ciple, dates  from  the  investigation  of  G-ustav  Rose  on  the  feldspathic  minerals  entering 
into  the  composition  of  rocks,  and  it  has  since  been  more  generally  applied  for 
establishing  subdivisions  than  any  other.  The  presence  or  absence  of  quartz,  the 
predominance  among  the  feldspathic  minerals  of  orthoclase,  oligoclase,  or  labrador, 
the  presence  of  augite  or  hornblende,  are  the  usual  points  of  issue,  even  in  the  most 
recent  attempts  at  classification.  The  high  value  of  mineralogy  as  a  basis  of  classi- 
fication cannot  be  denied.  But  its  exclusive  application  has  caused  the  combi- 
nation into  certain  groups,  of  such  rocks  as  from  a  geological  point  of  view  are 
widely  separated,  while  it  has  given  rise  to  distinctions  in  cases  where  the  results  of 
geological  observation  would  demand  close  connection,  as  we  shall  have  occasion  to 
illustrate  in  the  following  pages,  with  reference  to  those  volcanic  rocks  which  are  com- 
posed of  hornblende  and  oligoclase.  Gradually,  those  differences  based  on  chemical 
composition,  not  capable  of  being  detected  by  the  eye,  and  the  knowledge  of  which 
could  only  be  obtained  after  chemistry  had  made  the  necessary  advances,  have  become 
an  object  of  scientific  research.  But  this  principle  has  pot  yet  been  used  to  any  great 
extent  for  classification.  It  can  easily  be  demonstrated  that,  when  exclusively  ap- 
plied, it  leads  to  a  systematical  arrangement  of  rocks  which  is  in  even  greater  contra- 
diction with  the  natural  mode  of  occurrence  than  when  the  same  is  based  upon 
'  mineral  composition  alone,  notwithstanding  the  fact  that  its  great  value  has  been  con- 
clusively demonstrated,  especially  by  the  important  results  which  Bunsen  obtained 
from  the  chemical  analysis  of  rocks,  and  which  mark  an  era  in  petrology.  To  com- 
bine granite,  quartzose  porphyry,  and  rhyolite  into  one  class,  because  they  resemble 
each  other  in  their  chemical  composition,  and  to  place  them  at  the  head  of  the  list 
because  containing  the  highest  amount  of  silica  observed  among  eruptive  rocks,  would 
be  to  take  no  regard  whatever  of  geological  facts.  Rhyolite  is,  mineralogically  and 
geologically,  far  nearer  related  to  trachyte  than  to  either  granite  or  quartzose  por- 
phyry ;  and  these  two  are  quite  distinct  from  each  other,  while  granite  is  closely  allied, 
by  gradual  passage,  to  syenite,  and  quartzose  porphyry  to  porphyrite. 

It  is  by  slow  degrees  only  that  we  can  hope  to  reach  a  more  scientific,  that  is, 
a  more  natural  system  in  this,  the  most  intricate  branch  of  descriptive  natural  sciences. 
The  natural  differs  from  the  artificial  system  in  this,  that  it  starts  from  the  application 


8  RICHTHOFEN THE   NATURAL   SYSTEM 

not  of  one  only  but  of  various  principles,  compares  and  weighs  the  results  obtained  by 
each  of  them,  and  accepts  them  as  final  only  when  perfectly  harmonizing  among  each 
other.  It  is  then  that  it  tries  to  determine  what  principles  are  most  available  for 
establishing  the  higher  orders,  and  which  for  the  subdivisions.  The  singular  complica- 
tion which  is  peculiar  to  the  classification  of  rocks,  is,  besides  the  reasons  already 
mentioned,  due  in  a  great  measure  to  the  fact  that  geology  combines  the  double  func- 
tions of  a  historical  and  an  inductive  science,  while  in  petrology  we  have  besides  the 
requirements  of  a  descriptive  natural  science.  The  natural  system  of  rocks  should 
therefore  be  based,  not  only  upon  the  entire  range  of  their  petrographical  characters, 
such  as  mineral  composition,  chemical  composition,  texture,  and  specific  gravity,  but 
also  upon  their  mode  of  origin  and  geological  occurrence.  Classification  of  objects 
and  classification  of  relations  are,  with  them,  closely  connected,  and  should  be  made 
to  assist  each  other. 

The  question  may  be  raised,  whether  a  natural  system  of  rocks  based  upon 
such  principles  can  be  established  at  all,  and  if  it  can,  whether  it  would  be  of  any  use 
for  the  advancement  of  science.  To  the  first  question,  the  answer  must  be  in  the 
negative,  as  far  as  sedimentary  rocks  are  concerned.  They  have  been  formed  by  a 
complexity  of  circumstances,  and  just  so  complex  and  infinite  in  variety  are  they,  in 
respect  to  chemical  and  mineral  composition  and  all  external  characters.  To  analyze 
in  detail  their  mode  of  origin,  and  the  sources  from  which  their  material  has  been 
derived,  transcends  the  faculty  of  human  intellect,  and  it  would  be  a  hopeless  task  to 
attempt  to  discover  any  laws  regulating  the  boundless  differences  of  their  composition. 
They  are  thus  debarred  from  natural  classification,  though  its  principles  may  be  applied 
imperfectly  to  the  establishment  of  some  general  groups.  We  arrive  at  similar  con- 
clusions in  regard  to  those  rocks,  the  sedimentary  origin  and  subsequent  mctamorphism 
of  which  can  be  proved.  Accidental  and  local  circumstances  have  played  as  conspic- 
uous a  part  in  their  first  deposition  as  was  the  case  in  regard  to  those  sedimentary 
rocks  to  which  the  term  metamorphic  has  not  been  usually  applied.  But  as  meta- 
morphic  processes  of  a  certain  nature  have  ordinarily  affected  extensive  tracts  of 
these  rocks,  and  similarly  pervaded  great  thicknesses  of  them,  the  local  differences  of 
their  action  having  been  apparently  more  in  degree  than  in  mode,  they  have  occa- 
sioned a  certain  similarity  of  effect  which  partly  conceals  the  original  differences  in 
the  composition  of  the  rocks  affected  ;  and  it  appears  that  the  differences  in  the  kind 
and  intensity  of  metamorphic  action,  though  recognizable  only  in  their  final  results, 
will,  when  better  known,  afford  a  convenient  principle  for  a  classification  which  may 
have  some  similarity  with,  but  not  the  full  requirements  of,  the  natural  system.  Tt  is 
different  with  those  rocks  which  on  the  surface  of  the  globe  appear  as  intrusive  or 
eruptive  masses.  Notwithstanding  their  infinite  variety  in  character  and  composition, 
they  are  connected  by  definite  relations  which  bring  their  elementary  composition 
even  within  range  of  mathematical  calculation.  Their  recurrence  in  the  most  widely 
separated  countries,  with  similar  external  character,  identical  chemical  composition, 
and  in  analogous  relative  order  of  succession,  is  another  distinguishing  feature  of 
eruptive  rocks.  For  these  reasons,  as  well  as  iu  virtue  of  other  peculiar  characters 

(46) 


OF   VOLCANIC   ROCKS. 

which  will  be  more  fully  mentioned  in  other  pages,  they  appear  to  owe  their  present 
positions  to  the  action  of  general  planetary  processes,  and  to  reveal  by  their  own 
nature  that  of  the  mineral  matter  participating  in  the  original  composition  of  the 
globe,  and  by  their  order  of  succession,  the  mode  in  which  the  same  is  arranged 
beneath  the  theater  of  those  changes  which  since  a  remote  period  have  been  taking 
place  on  its  surface.  Only  presumptive  evidence  can  be  adduced  in  favor  of  this 
common  and  yet  much  disputed  theory.  The  probability  of  its  approximating  the 
truth  could  hardly  be  better  established  by  any  other  evidence  than  by  the  proof  that 
all  eruptive  rocks  of  the  globe,  taking  their  historical  part  into  account,  are  capable 
of  being  brought  into  a  natural  system  ;  or,  to  express  it  more  correctly,  that  they 
form  among  themselves  a  natural  system,  the  laws  of  which  we  may  be  capable  of  dis- 
covering. Considering  this  in  its  widest  bearing,  as  embracing  all  the  definite  correla- 
tions of  eruptive  rocks,  and  being  indeed  their  philosophical  expression,  we  may 
expect  that  it  will  make  us  acquainted  with  the  history  of  one  great  feature  in  the 
development  of  the  globe.  Bearing  in  their  own  character  and  system  the  imprint  of 
their  origin,  the  eruptive  rocks  will,  by  their  nature  itself,  allow  well-founded  con- 
jectures as  to  the  interior  structure  and  composition  of  the  earth.  This,  then, 
together  with  a  more  perfect  understanding  of  everything  connected  with  the  agencies 
working  below  the  surface  of  the  globe,  would  be  the  philosophical  use  of  the  natural 
system  of  eruptive  rocks. 

Only  initiatory  steps  can  be  taken  at  the  present  time  towards  the  establish- 
ment of  this  system.  I  have  confined  myself  in  this  essay  to  an  attempt  at  classifying, 
in  a  way  as  natural  as  experience  will  allow,  the  "  volcanic  rocks,"  that  is,  the  eruptive 
rocks  of  Tertiary  and  Post-Tertiary  ages.  The  term  "volcanic  rocks,"  has  been  chosen, 
because  the  rocky  matter  ejected  by  active  volcanoes  belongs  altogether  to  this  class, 
and  because  almost  every  kind  of  rock,  generated  by  eruptive  activity  during  the 
period  indicated,  has  partially  forced  its  way  through  volcanic  vents.  We  must  keep 
the  two-fold  mode  of  occurrence  of  volcanic  rocks  clearly  separated  in  our  minds.  We 
see  them  at  the  present  day  flowing  from  craters  in  the  shape  of  lava,  or  being  thrown 
out  as  scoria  and  rapilli  ;  and  there  is  abundant  evidence  that  their  mode  of  origin 
was  very  frequently  the  same  in  past  ages.  But  in  other  places,  it  is  perfectly  clear  that 
volumes  of  matter  of  the  same  kind  have  been  forced  to  the  surface  through  extensive 
fissures  and  accumulated  above  them  in  elongated  ranges,  when  the  origin  of  the  out- 
breaks cannot  be  ascribed  to  volcanic  activity.  These  eruptions  are  evidently  similar' 
in  nature  to  those  by  which  the  greater  part  of  the  granite,  syenite,  or  quartzose  por- 
phyry ascended  to  the  surface  in  ancient  times.  We  shall  distinguish  the  two  modes 
of  eruptions,  for  the  use  in  this  present  paper,  as  "volcanic  eruptions"  and  "massive 
eruptions,"  and  shall  dwell  in  the  sequel  more  fully  on  the  difference  between  both 
manifestations  and  their  probable  causes.  Then,  too.  the  reasons  will  be  mentioned 
which  justify  the  uniting  of  all  recent  eruptive  rocks  into  one  separate  class. 

No  other  class  of  eruptive  rocks  offers  greater  difficulties  for  a  systematical 
arrangement,  as  none  presents  throughout  so  great  a  number  of  varieties,  and  so  many 
accidental  modifications  of  texture  and  mineral  composition.  On  the  other  hand,  their 

(47) 


10  RICHTHOFEN THE    NATURAL   SYSTEM 

classification  is  of  especial  importance,  as  it  furnishes  the  key  for  deciphering  the 
natural  system  of  the  ancient  eruptive  rocks.  Volcanic  rocks  issue  from  volcanic 
vents  under  our  very  eyes,  and  the  record  of  the  history  of  those  which  have  origi- 
nated in  past  ages,  as  preserved  in  their  geological  relations,  is  far  more  distinct  than 
it  is  in  regard  to  their  ancient  predecessors.  No  doubt  exists  in  the  mind  of  any  ob- 
server in  respect  to  the  eruptive  origin  of  all  basalt  ;  while  in  regard  to  granite,  very 
different  views  are  entertained  by  distinguished  geologists,  and  supported  by  weighty 
arguments.  In  the  former  case,  we  have  conclusive  evidence,  while  in  the  second 
speculation  has  a  wider  scope.  The  knowledge  of  volcanic  rocks  will,  for  this  reason, 
facilitate  the  correct  interpretation  of  the  nature  of  rocks  generated  in  remote  ages. 

Volcanic  rocks  are  widely  spread  over  the  face  of  the  globe.  It  would  be  an 
object  of  great  interest  to  lay  down  their  geographical  distribution  on  maps,  and  to 
explore  the  laws  by  which  this  distribution  has  been  governed.  This  has  been  tried 
in  regard  to  active  volcanoes,  and  the  importance  and  interest  attaching  to  the  results 
obtained  are  such  as  to  have  given  rise  at  once  to  speculations  as  to  the  connection 
between  volcanoes  and  other  phenomena.  The  value  of  those  results  would  be  in- 
creased, if  to  the  active  craters  were  added  the  vastly  greater  number  of  those  extinct 
volcanoes,  the  mode  of  preservation  of  which  still  allows  us  to  recognize  their  former 
nature.  Even  then,  however,  the  maps  would  convey  but  a  remote  idea  of  the  gen- 
eral distribution  of  volcanic  rocks;  the  areas  comprised  by  which  should  be  marked 
out  with  proper  distinction  of  their  main  subdivisions.  Besides'an  immediate  bearing 
on  more  special  geological  questions,  the  knowledge  of  these  subjects  promises  to  be 
of  high  value  for  that  entire  department  of  geological  science,  by  which  the  latter  is 
most  closely  connected  in  scope  with  the  science  of  physical  geography.  Many  weighty 
problems,  such  as  the  causes  of  the  present  direction  and  extent  of  mountain  ranges, 
of  the  outlines  of  continents,  of  the  position  and  shape  of  groups  of  islands,  of  the 
secular  oscillations  of  the  surface  of  the  globe,  and  many  other  questions,  appear  to  be 
intimately  connected  with  the  geographical  distribution  of  volcanic  rocks  and  their 
mutual  geological  relations.  The  deduction  from  the  latter  of  definite  laws  appears 
to  be  the  initiatory  step  towards  understanding  the  laws  of  eruptive  activity  of 
remote  times,  and,  thereby,  towards  establishing  a  chapter  in  the  history  of  the  globe, 
which  is  among  the  obscurest  and  least  understood. 

The  following  classification,  in  which  existing  names  are  retained,  as  nearly  as 
could  be  done  with  convenience,  is  chiefly  founded  on  observations  made  in  the  Carpa- 
thians and  in  the  States  of  California  and  Nevada.  In  respect  to  the  variety  and  the 
distinctness  of  the  mutual  relations  of  the  volcanic  rocks,  these  two  countries  are  hardly 
surpassed  by  any  in  which  this  subject  has,  up  to  this  time,  been  scientifically  investi- 
gated. Until  recently,  all  volcanic  rocks,  at  least  those  of  more  frequent  occurrence, 
were  comprehended  in  the  terms  :  trachyte,  phonolite,  trachydolerite,  dolerite,  basalt ; 
while,  besides,  separate  names  were  used  to  distinguish  modifications  of  texture,  such 
as  pumice-stone,  obsidian,  pearlite ;  or  varieties  somewhat  more  distinct  in  point  of  min- 
eral composition,  such  as  leucitophyre.  The  classification  as  given  by  Al.  von  Humboldt, 
in  the  fourth  volume  of  the  "Cosmos,"  may  be  considered  as  having  represented  the 

(48) 


OF   VOLCANIC   ROCKS.  11 

most  advanced  stage  of  the  science  a  few  years  ago,  because  it  was  guided  by  a  definite 
principle ;  though  when  compared  with  contemporaneous  works  on  petrology,  it  gives, 
on  the  other  hand,  a  remarkable  illustration  of  the  great  difference  in  the  way  in  which 
the  same  names  have  been  applied.  Since  then,  the  name  "  rhyolite  "  (Richthofen, 
Studien  aus  den  ungarisch-siebenbiirgischen  Trachytgebirgen,  in  Jahrbuch  der  K.  K. 
geologischen  Reichsanstalt  in  Wien,  Yol.  XI  [1860], .pp.  153-277)  has  been  introduced 
for  a  very  distinct  class  of  volcanic  rocks.  Adding  this  to  the  previous  list,  there 
results  a  number  of  names  which,  in  geological  treatises,  are  either  grouped  com- 
pletely at  random,2  or  in  an  arbitrary  order,  or  arranged  by  artificial  principles,  when 
the  whole  classification  ordinarily  comprises  volcanic  and  ancient  eruptive  rocks  pro- 
miscuously. In  order  to  establish  a  more  natural  system,  we  have,  not  to  make  groups, 
but  to  find  them.  Dropping  all  of  those  d  priori  principles  which  may  be  conceived 
having  an  artificial  basis,  we  must  endeavor  to  discover  whether  any  great  divisions 
are  established  by  nature  herself,  and  if  so,  of  what  character  they  are.  We  may 
then  apply,  as  second  in  the  order  of  their  importance,  those  results  which  are  obtained 
in  the  laboratory  or  geological  cabinet,  for  defining  and  subdividing  those  groups. 
Most  of  the  natural  divisions  which  may  be  derived  from  geological  observation,  coin- 
cide essentially  with  those  based  on  artificial  principles,  but  are  more  naturally 
limited  as  regards  each  other.  «Each  of  them  has  its  own  more  or  less  independent 
part  in  the  architecture  of  mountain  ranges,  and  a  distinct  geological  age  in  reference 
to  the  other  groups.  Each  of  them  comprehends  a  series  of  rocks,  which,  besides,  are 
closely  connected  by  the  relations  of  their  petrographical  characters,  chemical  compo- 
sition, texture,  specific  gravity,  and  other  properties.  The  test  of  the  natural  founda- 
tion and  general  validity  of  these  groups  will  be  their  recurrence,  with  mutual  rela- 
tions unchanged,  in  different  parts  of  the  globe,  of  which  test  we  are  never  to  lose 
sight. 

The  following  is  the  classification,  the  approach  of  which  to  a  natural  system 
of  volcanic  rocks,  I  will  endeavor  to  set  forth  in  the  course  of  this  paper : 

ORDER  FIRST:  Rhyolite. 

Family  1.  Nevadite,  or  granitic  rhyolite. 

2.  Liparite,  or  porphyritic  rhyolite. 

3.  Rliyolite proper,  or  lithoidic  and  hyaline  rhyolite. 

ORDER  SECOND:  Trachyte. 

Family  1.  Sanidin-trachyte. 
2.   Oliyodase-trachyte. 


'l  That  this  is  even  done  in  books  of  the  highest  standard,  may  be  seen  by  reference  to  one  so  prominent  as  LyelVs 
Elements  of  Geology.  The  following  is  the  order  of  names  of  which,  under  the  head  of  "  volcanic  rocks,"  definitions  are 
given  (p.  592,  pp.  of  Cth  Am.  cd.,  1866)  :  Basalt,  augite  rock,  trachyte,  trachytic  porphyry  (in  connection  with  which  the 
name  "  andesite  "  is  mentioned),  clinkstone,  greenstone,  porphyry,  amygdaloid,  lava,  scoriae  or  pumice,  volcanic  tuff  or  trap- 
tuff,  agglomerate,  laterite. 

(49) 


12  RICHTHOFEN  —  THE    NATURAL    SYSTEM 

ORDER  THIRD:  Propylite. 

Family  1.    Quarlzose  propylife. 

2 .  Hornblendic  propylite. 

3.  Augitic  propylite. 

ORDER  FOURTH:  Andesite. 

Family  1.  Hornblendic  andesiie. 

2.  Augitic  andesite. 

ORDER  FIFTH:  Basalt. 

Family  1 .  Dolerite. 
"2.  Basalt. 

3.  LewxLophyre. 

ORDER  FIRST — RIIYOLITE. 

The  name  "  rhyolitc"  was  proposed,  early  in  I860,8  for  certain  rocks  fre- 
quently occurring  on  the  southern  slope  of  the  Carpathians,  and  distinguished,  in  min- 
eral character,  from  trachyte,  which  they  otherwise  resemble,  by  the  presence  of  quartz 
as  an  essential  ingredient,  and  an  almost  infinite  variety  of  texture.  Beudant4  had, 
long  before,  described  certain  varieties  of  these  rocks  a*s  porphyre  trachytique,  pumice- 
stone,  pearlite,  etc.  In  1861,  the  name  "  liparite"  was  proposed  by  J.  Roth5  for  rocks 
of  similar  nature  occurring  on  the  Lipari  Islands.  The  term  ' '  rhyolite,"  however, 
being  of  prior  date,  has  since  been  almost  generally  adopted,  among  others  by  F.  v. 
Hochstetter,  for  rocks  from  New  Zealand,  by  C.  Peters,  G.  Stache,  and  others  for  those 
of  Hungary  and  Transylvania,  by  Ferd.  Zirkel  for  those  of  Iceland,  by  B.  v.  Cotta  as 
a  general  term  in  his  "  Gesteinslehre."  The  word  "  rbyolite"  is  designed  to  express 
one  of  the  prominent  features  of  these  rocks.  It  is  this :  that  their  chief  varieties 
have  the  appearance,  as  it  were,  of  natural'glasses,  and  bear  evidence,  more  than  any 
other  rocks  do,  to  the  unpracticed  eye,  of  having  been  flowing  in  a  viscous  state. 

Mode  of  Geological  Occurrence. — Rhyolite  has  had  its  distinct  epoch  of  eruption  in 
relation  to  other  volcanic  rocks.  Wherever  it  occurs  it  may  be  easily  proved  to  have 
been  of  more  recent  origin  than  either  propylite,  andesite,  or  trachyte,  but  to  have 
preceded  basalt  in  age.  As  to  its  geographical  distribution,  it  is  confined  to  the  im- 
mediate neighborhood  of  one  or  all  of  those  antecedent  rocks,  and  occurs  ordinarily 
within  their  very  limits.  Until  lately,  not  much  attention  had  been  paid  to  it.  But 
since  the  establishment  of  the  name,  rhyolite  has  been  found  to  be  widely  distributed, 
though  always  occupying  a  subordinate  position.  In  Hungary  it  usually  skirts  the 
lower  part  of  the  flanks  of  andesitic  ranges,  forming  hillocks  and  ridges  of  little 
elevation,  filling  depressions,  and  issuing  in  currents  from  fractures,  and,  in  general, 
giving  evidence  of  its  entire  dependency  on  the  places  of  previous  eruptions. 
The  greater  portion  of  the  rhyolite  has,  in  that  country,  been  evidently  ejected 
by  volcanic  activity.  It  appears  that  the  same  may  be  said  in  regard  to  the 

*  Loc.  Cit.  *  Voyage  en  Hongrie.    Paris,  1820.  5  J.  Both,  die  Gesteins-analysen.    Berlin,  1861. 

(50) 


OF    VOLCANIC   ROCKS.  13 

rhyolitic  rocks  of  New  Zealand,  St.  Paul,  and  Iceland.  But  it  has  not  been 
so  generally  the  case  on  the  western  coast  of  America.  The  volcano  of  Lassen's 
Peak,  and  the  environs  of  Mount  Helena,  in  California,  present  grand  instances 
of  a  volcanic  origin  of  rhyolite.  But  in  the  adjoining  *  State  of  Nevada  it  ap- 
pears to  have  been  extensively  brought  to  the  surface  by  massive  eruptions.  It  is  of 
unusually  frequent  occurrence  along  the  eastern  slope  of  the  Sierra  Nevada,  and  far- 
ther east  in  the  Great  Basin.  Ranges  of  hills  are  there  completely  built  up  of  rhyo- 
litic rocks,  not  always  in  as  close  proximity  to  such  as  are  antecedent  to  them  in  age, 
as  is  the  case  in  the  Carpathians. 

It  is  one  of  the  characteristic  features  of  rhyolite,  that  it  presents,  more  than 
any  other  rock  does,  signs  of  having  been  in  a  state  of  what  Daubree  has  called 
"aqueous  fusion,"  or  the  fusion  of  its  mass  by  solution,  under  great  pressure,  in  super- 
heated water.  Another  peculiarity  is  the  circumstance  that  the  eruptions  of  rhyolite, 
whether  massive  or  volcanic,  bear  evidence  of  having  been  generally  accompanied  by 
extremely  violent  solfataric  action,  which  probably  surpassed,  on  an  average,  that  con- 
nected with  the  ejection  of  other  volcanic  rocks.  This  action  appears  to  have  been 
one  of  the  chief  agents  in  the  formation  of  the  rich  silver-bearing  veins  of  Hungary, 
as  well  as  of  some  in  Mexico,  and  to  have  also  been  peculiarly  characterized  by  the 
occurrence  of  an  unusually  large  amount  of  fluorine  and  chlorine  among  the  escaping 
gases. 

Mineral  Composition. — Rhyolite  may  be  concisely  defined  as  trachyte  with  an 
addition  of  silica,  not  chemically  combined,  and  which  is  either  segregated  into  crystals 
of  quartz,  or  dissolved  in  the  rock,  and  then  no  longer  recognizable  to  the  eye.  It  is, 
owing  to  the  high  proportion  of  silica  entering  into  its  composition,  the  representative 
of  granite  among  volcanic  rocks.  Chemically,  it  is  its  complete  counterpart,  as  far  as 
ascertained  by  analysis,  and  even  in  outward  appearance  certain  varieties  of  rhyolite 
offer  at  first  sight  a  striking  similarity  with  granite,  though  closer  observation  will  at 
once  reveal  well  marked  differences  between  both.  Rhyolite  outrivals  any  other  rock 
in  respect  to  the  truly  astonishing  number  of  its  varieties,  which  are  chiefly  occasioned 
by  modifications  of  texture.  It  consists  in  general  of  a  paste,  with  or  without  minerals 
enclosed. 

The  paste,  chiefly,  is  liable  to  variation.  Its  colors  are:  white,  gray,  yellow, 
green,  red,  brown,  which  occur  in  all  manner  of  shades  ;  light  ones  prevail,  while 
perfect  black  has  not  been  met  with.  The  texture  is  as  varied  as  the  color.  First 
are  to  be  noticed  a  number  of  hyaline  varieties,  which  are  represented  by  obsidian, 
pumice-stone,  and  pearlite,  and  the  frequent  occurrence  of  which  is  a  peculiar  feature 
of  rhyolite.  Though  associated  with  volcanic  rocks  of  every  composition,  these  natu- 
ral glasses,  as  they  may  be  called,  decrease  in  relative  quantity  and  variety  with  the 
decrease  of  silica.6  Obsidian  when  having  the  composition  of  rhyolite,  offers  little 

6  There  is  no  better  example  of  tlie  artificial  principles  on  which  the  classification  of  rocks  lias  usually  been 
based  than  the  fact,  that  accidental  modifications  of  texture,  which  appear  to  result  chiefly  from  the  difference  of  the  condi- 
tions attending  either  the  fusion  or  the  cooling  of  the  mass,  have  been  considered  as  of  equal  value  with  other  differences  of 
the  greatest  importance ;  and  pumice-stone,  trachyte,  basalt  and  pearlite  have  been  considered  as  coordinate  subdivisions, 
even  long  after  classification  had  been  made  dependent  chiefly  on  mineralogical  principles. 

a  (51) 


14  RICIITHOFEN NATURAL   SYSTEM 

difference  in  character  from  that  which  is  a  modification  of  other  volcanic  rocks.  In 
regard  to  pumice-stone,  however,  Abich  has  proved  that  when  formed  of  the  material 
of  trachyte  or  andesite  it  has  rounded  pores,  and  ordinarily  a  green  tint ;  while  those 
varieties  which  have  the  composition  of  rhyolite,  excel  by  the  elongated  and  irregular 
shape  of  their  cavities,  which  are  enclosed  in  a  fine  tissue  of  fibres  of  silken  appearance 
and  white  color.  Between  both  kinds  of  pumice-stone  there  are  gradations  apparently 
dependent  as  to  their  character  on  the  amount  of  silica  entering  into  their  composition. 
Pearlitic  texture  is  peculiar  to  rhyolitic  rocks.  From  these  more  or  less  perfectly  glassy 
varieties  there  are  gradations  to  the  texture  of  enamel  and  porcelain,  and  to  a  certain 
cryptocrystalliue  texture  very  frequent  among  volcanic  rocks  in  general,  and  for  which 
we  may  apply  the  obsolete  term  "lithoid."  This  passes  into  the  microcrystalline,  and 
always  more  or  less  vesicular,  texture  of  trachyte. 

The  paste  constitutes  occasionally  alone  the  substance  of  the  rock.  But  these 
instances  are  rare.  More  frequently  it  contains  enclosed  mineral  substances  differing 
from  it  in  nature,  and  in  a  few  instances  these  accumulate  to  such  a  degree  as  almost 
to  exclude  the  paste.  Quartz  is  of  the  most  general  occurrence  among  those  which 
are  crystallized.  Sanidin  is  its  almost  unfailing  companion.  Oligoclase,  usually  of  a 
vitreous  variety,  and  black  mica,  are,  too,  among  the  usual  ingredients,  while  horn- 
blende is  generally  less  conspicuous.  Besides  these  minerals,  there  are  two  substances 
entering  accidentally  into  the  composition  of  rhyolite,  which  are,  however,  among  the 
characteristic  features  of  the  rocks  of  this  order.  One  of  them  consists  in  small  glob- 
ular grains,  from  the  size  of  a  pin-head  to  that  of  a  rifle  bullet,  called  "  sphaerolites  " 
by  Beudant.  They  have  a  radial  structure,  and  contain  ordinarily  a  small  crystal  of 
feldspar  in  the  center.  Certain  hyaline,  and,  in  a  greater  measure,  lithoid  varieties 
of  rhyolite  contain  them  in  large  quantity.  They  occur  also,  though  less  frequently, 
in  other  natural  glasses  not  of  rhyolitic  composition,  and  may  be  produced  artificially, 
by  allowing  molten  glass  to  cool  very  slowly  to  what  is  known  by  the  term  "  Reau- 
mur's porcelain."  The  second  formation  frequently  met  with  in  rhyolitic  rocks  are 
the  "lithophysae,"  consisting  in  larger  and  smaller  cavities  filled  by  a  substance 
strangely  inflated  by  some  gaseous  evolution  which  apparently  originated  in  the  mat- 
ter itself  (Richthofen,  1.  c.).  They  constitute  sometimes  nearly  the  entire  mass  of 
the  rock. 

The  endless  varieties  of  rhyolite  appear  to  be  due  to  the  susceptibility  of  the  fluid 
mass  to  be  influenced  by  accidental  circumstances  to  which  it  may  have  been  exposed, 
partly  before  being  ejected,  and  partly  during  the  process  of  solidifying.  A  consid- 
erable influence,  which,  however,  has  not  yet  been  investigated,  is  probably  exercised 
by  the  difference  in  the  amount  of  water  which  entered  into  the  composition  of  the 
molten  mass,  and  partly  expanded  to  steam  in  the  instant  of  ejection.  The  vesicular 
inflation  proper  to  trachytic  texture,  the  spongy  inflation  of  pumice-stone,  and  the 
concentric  separation  of  infinitely  fine  laminae,  as  is  often  shown  in  perfect  pearlitic 
texture,  are  probably  three  different  modes  of  manifestation  of  one  slightly  varied 
cause,  which  may  most  likely  be  found  in  the  conversion  of  water  into  steam,  which 
participated  in  the  composition  of  the  molten  mass. 

(52) 


OF   VOLCANIC   EOCKS.  15 

Difference  of  Rhyolite  from  other  Rocks  nearly  related  to  it. — Several  varieties  of 
rhyolite  bear  so  close  a  resemblance  to  other  rocks,  that  some  mention  must  be  made 
of  their  distinguishing  features.  Rhyolite  may  be  easily  distinguished  from  granite.  The 
varieties  by  which  it  mostly  approaches  the  same  are  those  which  contain  crystals  of 
quartz,  feldspar  and  mica  in  unusually  large  proportion  and  size.  But  in  the  case  of  rhy- 
olite, the  paste  in  which  they  are  imbedded  is  never  wanting.  Moreover,  the  orthoclase 
and  oligoclase  are  of  the  vitreous  varieties,  and  quartz  is  present  either  in  crystals  or  in 
rounded  crystalline  grains,  while  in  granite  it  usually  permeates  the  interstices  between 
the  other  component  minerals.  Much  closer  is  the  affinity  which  certain  other  varie- 
ties of  rhyolite  bear  to  quartzose  porphyry,  especially  those  which  have  a  paste  of 
homogeneous  appearance  containing  no  crystals  but  those  of  quartz  inclosed.  Geo- 
logical observation  will  never  fail,  in  such  instances,  to  establish  the  nature  of  the 
questionable  rock,  as  it  will  show  its  association  either  with  true  rhyolite  or  with 
true  porphyry.  The  same  test  has  to  be  applied  occasionally  with  respect  to  some 
other  varieties  which  contain  the  silica  equally  diffused  through  the  paste,  and  bear  a 
close  resemblance  to  trachyte.  In  this  instance,  however,  even  geological  observations 
will  sometimes  fail  to  determine  the  exact  position.  There  is  a  gradual  passage  in 
character  between  every  two  nearly  related  rocks,  such  as  rhyolite  and  trachyte,  or 
granite  and  syenite,  and  it  frequently  happens  that  either  name  may  be  used  with 
equal  right. 

Subdivisions.- — In  establishing  the  subdivisions  of  most  orders  of  eruptive  rocks, 
mineral  composition  affords  a  principle,  not  only  the  most  convenient  for  practical 
application,  but  one  that  answers  well  the  requirements  of  the  natural  system,  when 
made  subordinate  in  value  to  those  higher  principles  which  determine  the  limits  of 
classes  and  orders.  In  the  case  of  rhyolitic  rocks,  however,  it  is  not  as  applicable  as 
in  that  of  other  orders.  They  should,  from  this  point  of  view,  be  subdivided  into 
those  which  contain  quartz  and  those  which  are  devoid  of  it,  or  into  such  as  carry 
sanidin  and  such  as  contain  both  sanidin  and  oligoclase.  But,  since  rhyolite  of  any 
certain  chemical  composition  may  contain  its  surplus  of  silica,  either  visibly  segregated  in 
crystals  of  quartz,  or  dissolved  in  the  mass  of  the  rock,  and  as  the  case  may  be  similar 
iu  regard  to  the  occurrence  of  either  species  of  feldspar,  the  application  of  this  princi- 
ple would  lead  us  to  combine  into  one  group  quartzose  rocks  differing  considerably 
among  themselves  as  to  the  proportion  of  silica  they  contain,  while  another  group 
might  comprehend  rocks  of  virtually  the  same  nature  as  those  of  the  first,  and  differing 
from  them  only  accidentally  in  external  character.  More  natural  subdivisions  of  rhy- 
olitic rocks  are  obtained  by  taking  as  a  basis  of  classification  their  difference  in  texture, 
which  either  approaches,  to  a  certain  degree,  that  of  granite  or  is  porphyritic  or  hya- 
line. It  is  a  singular  fact,  and  one  difficult  of  explanation,  that  rhyolite,  at  every  place 
where  it  has  been  hitherto  observed,  presents,  either  solely  or  chiefly,  one  of  those 
three  modes  of  texture.  Lassen's  Peak,  for  instance,  presents  the  granitic  variety 
almost  exclusively.  Sonoma,  in  California,  and  the  Tokay  Mountains  in  Hungary, 
only  the  hyaline,  and  other  places  exclusively  the  porphyritic  varieties.  This  circum- 
stance, which  is  peculiar  to  none  but  rhyolite  among  eruptive  rocks,  indicates  the 


16  RICHTHOFEN NATURAL   SYSTEM 

dependency  of  the  mode  of  texture  upon  deep-seated  influences  which  acted  at  the  very 
source  of  the  eruptive  matter,  and  produced  a  certain  molecular  condition  of  the  latter, 
varying  at  each  locality.  Some  light  on  this  subject  may  be  expected  from  minute 
geological  observation,  accompanied  by  exact  chemical  and  microscopical  researches. 

The  following  are  the  subdivisions  which  may  be  distinguished  in  regard  to  the 
texture  : 

Fam.  \st.  Nevaditeor  Granitic  Rhyolite. — The  name  "  Nevadite  "  is  derived  from 
that  of  the  State  of  Nevada,  where  these  rocks  have  been  first  met  with  in  larger 
accumulations.  The  local  derivation  may  answer  in  this  instance,  as  granitic  rhyolite 
is  little  known  from  other  countries,  excepting  the  neighboring  California.  In  the 
Carpathians,  it  occurs  isolated  in  Transylvania,  but  by  no  means  as  characteristic  as  in 
Nevada.  The  name  "  granitic  rhyolite  "  is  designed  to  indicate  the  general  resem- 
blance of  these  rocks  to  granite,  which  is  conspicuous  in  boulders,  or  on  large  exposed 
faces,  but  disappears  on  closer  examination.  It  is  chiefly  produced  by  the  similarity 
in  color,  which  is  of  light  shades  of  gray  and  red  in  Nevadite,  and  by  some  affinity 
in  mineral  composition.  Nevadite  contains  crystals  of  quartz  in  large  proportion  ;  the 
corners  are  usually  rounded,  and  the  quartz  itself  cracked,  like  glass  when  rapidly 
cooled.  Sanidin  occurs  in  crystals  of  larger  size  than  oligoclase,  sometimes  of  an  inch 
in  diameter.  The  crystals  of  both  are  often  cracked  throughout  their  mass,  and 
rounded  at  the  corners.  Black  mica  and  hornblende  are  ingredients  of  frequent  occur- 
rence. These  minerals  are,  in  more  or  less  quantity,  enclosed  in  a  paste  which  is 
probably  a  partially  microcrystalline,  and  partially  amorphic  aggregation  of  the  same 
ingredients,  and  has  a  highly  vesicular  texture,  rendering  it  rough  to  the  touch,  even 
more  so  than  is  the  case  with  trachyte.  Geologically,  Nevadite  appears  to  have  been 
produced  as  frequently  by  volcanic  activity  as  by  massive  eruptions.  An  interesting 
occurrence  is  that  at  Lassen's  Peak,  in  California,  where  it  was  discovered  by  Prof.  W. 
H.  Brewer  and  Mr.  Clarence  King. 

Fam.  2d.  Liparite,  or  Porphyritic  Rhyolite. — The  name  "Liparite,"  which  was  pro- 
posed by  J.  Roth  for  this  whole  class,  may  be  conveniently  retained  for  those  varieties 
of  rhyolite  which  approach  quartzose  porphyry  in  character,  as  they  appear  to  occur 
on  the  Liparic  Islands,  either  solely  or  at  least  in  larger  proportion  than  other  varieties. 
They  consist  of  a  paste  which  has  a  similar  texture  to  that  of  quartzose  porphyry,  and 
incloses  crystals  either  of  quartz  only,  or  of  quartz  and  sanidin,  or  of  quartz,  sanidin, 
oligoclase,  and  black  mica,  or  of  one  or  both  kinds  of  feldspar,  without  quartz  being 
present.  The  crystals  have  sharp  corners  and  are  seldom  cracked  ;  oligoclase  is  rarely 
of  the  vitreous  variety.  Typical  varieties  of  the  rocks  of  this  family  occur  largely  in 
the  hills  of  Bereghszasz  in  Hungary. 

Fam.  3d.  Rhyolite  proper,  or  Hyaline  Rhyolite. — The  extensive  range  of  varieties 
afforded  by  all  manner  of  modifications  of  hyaline  texture  are  a  peculiar  feature  of 
rhyolite,  distinguishing  it  from  any  other  eruptive  rock.  In  outward  appearance  they 
remind  one  of  artificial  glasses  cooled  under  the  most  varied  conditions.  Obsidian, 
pumice-stone,  and  pearlite,  constitute  but  a  small  portion  of  the  varieties  occurring  ; 

(54) 


OF   VOLCANIC   ROCKS.  17 

others  resemble  enamel  and  porcelain,  or  present  appearances  which  are  difficult  to 
describe.  The  occurrence  of  sphaerolites  and  lithophysos  add  to  the  variety  of  their 
aspect.  Another  feature  peculiar  to  the  rocks  of  this  family  consists  in  a  foliated 
structure,  the  folia  being  often  thinner  than  paper,  and  presenting  an  endless  variety 
of  color  and  modifications  of  texture.  Pearlite  alone  does  not  participate  in  the  pecu- 
liar form  of  foliated  structure. 

All  these  varieties  again  contain,  enclosed,  all  the  different  minerals  before  men- 
tioned, or  only  a  few  of  them,  or  they  enclose  no  foreign  substances  at  all.  The  rocks 
of  this  family  are  chiefly  of  purely  volcanic  origin,  but  in  some  instances  currents  of 
them  appear  to  have  been  ejected  through  crevices  in  the  older  volcanic  rocks. 

ORDER  SECOND — TRACHYTE. 

The  name  "trachyte"  was  first  used  by  Hauy,  in  his  academic  lectures,  to 
designate  the  well-known  volcanic  rocks  composing  the  Drachenfels  on  the  Rhine. 
But  it  did  not  come  into  general  use,  until  Beudant,  the  pupil  of  the  former,  intro- 
duced the  name  into  geological  literature,  by  his  justly  celebrated  work  "Travels 
through  Hungary,"  a  book  which  abounds  in  sagacious  observations  on  the  subject  of 
volcanic  rocks.  Beudant  extended,  however,  the  application  of  the  name  over  a  much 
wider  range  than  his  teacher  had  done.  Several  years  later,  in  1835,  L.  v.  Buch 
introduced  into  literature  the  name  ''  andesite,"  designating  by  it  certain  dark- 
colored  rocks,  which  were  then  known,  especially  through  the  collections  of  Al.  v. 
Humboldt  and  Boussingault,  to  enter  largely  into  the  composition  of  the  volcanic  por- 
tions of  the  South  American  Andes.  These  rocks,  which  form  obviously  a  part  of 
those  which  Beudant  had  comprised  under  the  name  "  trachyte,"  were  supposed  to 
be  particularly  distinguished  by  containing  a  peculiar  species  of  feldspar  which  Abich 
(in  1840)  called  "  andesine."  When,  however,  a  few  years  later,  this  mineral  was  no 
longer  considered  to  be  a  separate  mineralogical  species,  and  its  name  was  dropped, 
that  of  "  andesite  "  became  obsolete  with  it,  notwithstanding  its  prior  origin.  Thence- 
forth, the  range  of  the  varieties  of  volcanic  rocks  comprised  in  the  term  "  trachyte  " 
has  been  even  more  enlarged  than  Beudant  had  proposed  in  his  dissertation.  Eruptive 
rocks,  widely  differing  in  nature — in  fact  nearly  all  those  of  tertiary  and  post-tertiary 
age,  with  the  exception  of  basalt — have  been  united  in  it.  But  by  no  author,  proba- 
bly, was  the  application  of  the  name  as  much  extended  as  by  Al.  von  Humboldt.7 
On  the  other  side,  however,  he  was  the  first,  by  the  establishing  of  numerous  sub- 
divisions, to  draw  attention  upon  the  necessity  of  using  separate  names  for  more  lim- 
ited ranges  of  varieties.  Recently,  B.  von  Cotta,  J.  Roth,  and  others,  have  tried  to 
demonstrate,  how  little  reason  there  had  been  for  dropping  the  term  "andesite." 
They  re-introduced  it  into  petrology;  but,  in  drawing  the  limits  between  andesite  and 
cognate  groups  by  principles  of  artificial  classification,  they  used  the  name  in  a  sense 
differing  to  some  extent  from  that  in  which  it  has  been  applied  in  the  following  pages. 

7  Cosmos,  vol.  iv.     The  first   four  orders  of  trachyte  were  proposed  by  G.  Rose,  the  other  two  were  added  by 
Humboldt. 

(55) 


18  RICHTHOFEN — NATURAL   SYSTEM 

There  is  plainly  indicated,  from  a  geological  point  of  view,  the  existence  of  two  large 
groups  within  the  limits  of  Beudant's  "  trachyte."  The  two  existing  names,  "  trachyte  " 
and  "  andesite,"  may  conveniently  be  used  for  their  designation,  since  those  rocks, 
for  which  either  of  the  two  names  was  first  introduced,  are  indeed  the  types  of  the 
two  natural  groups. 

Mode  of  Geological  Occurrence. — The  trachytic  rocks  have  had  their  independ- 
ent epoch  of  eruption  in  every  volcanic  country.  They  preceded  rhyolite  and  basalt 
in  age,  and  were  posterior  to  the  ejection  of  propylite  and  andesite.  The  trachytic 
epoch  was  usually  of  long  duration.  In  many  localities,  its  later  part  blended  with 
the  earlier  of  the  rhyolitic  epoch,  which  is  manifest  by  the  alternate  emission  of  tra- 
chytic and  rhyolitic  matter,  during  that  time  which  was  intermediate  between  the  epochs 
of  the  ejection  of  the  principal  bulk  of  either  of  them.  As  regards  geographical  dis- 
tribution, trachyte  is  as  much  dependent  upon,  and  as  closely  allied  to,  the  preexisting 
masses  of  propylite  and  andesite,  as  is  the  case  with  rhyolite.  It  towers  up  in  peaks, 
cones,  and  ridges,  which  are  distinguished  by  their  rugged  outlines,  and  rest,  in  the  ma- 
jority of  cases,  upon  the  summits  or  the  flanks  of  the  ranges  composed  of  those  older 
volcanic  rocks.  But  there  are  also  numerous  instances  when  these  may  be  seen  to  be 
accompanied,  at  some  distance,  by  trachytic  outbursts,  when  a  cursory  examina-. 
tion  might  make  the  latter  appear  to  occupy  an  independent  position.  Trachyte  does 
probably  not  compose,  by  itself,  any  extensive  mountain  ranges,  and  it  remains,  in 
general,  greatly  inferior  in  bulk  to  andesite.  In* Europe,  its  outbreaks  were  scattered 
and  isolated,  and,  though  they  have  been  quite  numerous,  the  aggregate  quantity  of 
trachytic  rocks  is  not  considerable.  They  occur  in  Hungary  and  Transylvania,  on  the 
Lower  Rhine,  in  Central  France,  in  the  Grecian  Archipelago,  and  in  other  parts  of 
that  continent,  as  well  as  in  the  adjacent  portions  of  Asia.  Specimens  of  trachyte, 
owing  to  their  beauty  and  varied  aspect,  are  usually  much  more  numerous  in  geological 
cabinets  than  those  of  andesite — a  fact  which  has  frequently  occasioned  some  miscon- 
ception regarding  the  relative  proportion  and  importance  of  trachyte  and  andesite 
among  volcanic  rocks. 

In  the  structure  of  the  North  American  Andes,  trachyte  takes  a  more  import- 
ant part  than  in  Europe.  A  continuous  range  of  it,  at  least  ten  miles  in  extent,  and 
forming  rugged  crests,  encircles  the  Washoe  Mountains  to  the  east,  in  the  shape  of  a 
crescent.  Trachyte  rests  there  on  propylite,  and  its  ejection  has  probably  had  an 
intimate  connection  with  the  formation  of  the  Comstock  Lode.  Other  accumulations 
of  similar  extent  may  be  noticed  at  Esmeralda,  on  the  eastern  slope  of  the  Sierra 
Nevada,  around  Red  Rock  Canon,  south  of  Walker's  Pass,  in  the  surroundings  of  Lake 
Tahoe  and  Sierra  Valley,  and  at  other  places  east  and  west  of  the  Sierra  Nevada.  In 
the  northern  provinces  of  Mexico,  trachyte  is  known  to  occur  quite  extensively.  At 
all  the  places  mentioned,  the  greater  part  of  its  bulk  bears  evidence  of  having  been 
brought  into  its  present  position  by  massive  eruptions,  while  traces  of  extinct  trachytic 
volcanoes  are  scarcely  wanting  at  any  of  them.  Trachyte  is  still  being  ejected  by  a 
number  of  active  volcanoes.  Among  them  may  chiefly  be  mentioned  those  of  Central 
America,  the  greater  part  of  the  modern  lava  of  which  has  been  proved  to  have  the 

chemical  and  mineral  composition  of  trachyte. 
(56) 


OP    VOLCANIC   ROCKS.  19 

Mineral  Composition. — Trachyte  is  only  inferior  to  rhyolite  in  the  number  of  its 
varieties.  Yet  its  chemical  composition  is  as  simple  as  that  of  the  latter,  and  ranges 
within  as  definite  limits.  The  rock  appears  to  contain,  on  an  average,  from  60  to  65 
per  cent,  of  silica,  and  is  in  this  respect,  as  in  others,  next  allied  to  rhyolite.  The 
difference  between  the  rocks  belonging  to  both  orders,  having  its  fundamental  cause 
in  the  chemical  composition,  manifests  itself  externally  in  certain  differences  of  charac- 
ter, among  which  may  be  mentioned  :  the  absence  of  quartz  among  the  essential 
ingredients  of  trachytic  rocks,  which  probably  contain  no  free  silica  at  all  ;  the  usual 
predominance  in  them  of  oligoclase  over  sanidin ;  the  larger  proportion  in  which  horn- 
blende participates  in  their  mineral  composition,  and  the  fact  of  their  specific  gravity 
exceeding  that  of  rhyolite.  Like  almost  all  volcanic  rocks,  trachyte  consists  of  a  paste 
in  which  are  imbedded  various  crystallized  minerals.  This  paste  is  of  various  colors, 
and  has  usually  a  more  or  less  vesicular  texture,  which,  by  its  property  of  imparting 
to  the  rock  a  certain  roughness  of  touch,  gave  origin  to  the  name.  Chief  in  order 
among  the  enclosed  minerals  are  sanidin,  oligoclase,  mica,  and  hornblende.  They  vary 
considerably  as  regards  their  relative  proportion,  and  therefrom  arises  a  number  of 
varieties  which  have  been  partly  distinguished  by  separate  names.  More  numerous 
varieties,  however,  are  occasioned  by  the  differences  of  texture.  Pumice-stone  is  of 
frequent  occurrence  among  the  latter,  but  it  never  exhibits  that  perfect  long-fibrous 
and  silken  nature  peculiar  to  it  when  being  a  variety  of  rhyolite.  Obsidian,  in  differ- 
ent grades  of  perfection,  is  no  unusual  modification  of  the  trachytic  masses,  and  it  is 
often  filled  with  sphaerolites,  while  no  pearlite  has  yet  been  found  having  the  chemical 
composition  of  trachyte.  Foliated  structure  may  frequently  be  met  with  ;  but  the 
folia  are  not  of  that  exquisite  fineness  which  is  peculiar  to  those  of  rhyolite.  The 
modes  of  texture  which  alternate  in  the  folia,  are  chiefly  obsidian,  pumice-stone,  and 
microcrystalline  varieties. 

Subdivisions. — It  appears  that  two  natural  groups  of  trachytic  rocks  may  be 
distinguished,  which  differ  at  the  same  time,  from  a  mineralogical  and  chemical  point 
of  view,  and  have  therefore  been  arrived  at  similarly  by  the  application  of  artificial 
principles.8  We  distinguish  with  B.  v.  Cotta  : 

8  I  may  here  remark  that  I  have  endeavored  to  retain  existing  names,  as  much  as  possible,  for  designating  the  orders 
and  families  distinguished  in  this  present  classification.  What  I  have  tried  to  establish  as  natural  group?,  may  therefore,  on 
account  of  the  similarity  of  nomenclature,  coincide  apparently,  in  many  cases,  with  the  groups  established  by  artificial 
principles,  and  named  in  accordance  with  them.  The  former  do  indeed  coincide  in  a  few  instances  with  the  latter ;  but  in 
the  majority  of  cases,  the  limits  of  the  application  of  the  names  differ  widely  when  established  from  the  two  points  of  view 
mentioned.  It  would  require  too  much  space  to  go  into  detail  on  this  point  in  regard  to  every  name.  I  will,  therefore, 
confine  myself  to  an  illustration  regarding  the  order  of  trachytic  rocks.  B.  v.  Cotta,  (with  several  other  authors)  unites 
under  the  name  of  oligoclase-trachyte  all  volcanic  rocks  consisting  chiefly  of  hornblende  and  oligoclase,  with  the  exclusion 
of  certain  dark-colored  rocks,  to  which  he  applies  the  name  andesite.  The  former  name,  if  used  in  the  meaning  of  that 
author,  comprises,  therefore,  our  order  of  propylite,  together  with  all  those  rocks  for  which  the  term  oligoclase-trachyte  has 
been  here  applied.  It  is  evident,  from  the  different  geological  positions  occupied  by  propylite  and  trachyte,  as  well  as  from 
the  distinct  petrographical  character  which  either  of  them  exhibits  when  occurring  in  large  accumulations,  how  much  their 
union  would  be  opposed  to  a  correct  representation  of  natural  relations.  It  is  obvious,  besides,  that  the  union  is  unprac- 
tical. It  would  actually  prevent  the  possibility  of  a  clear  and  simple  geological  description  of  those  countries  in  which 
propylite  and  trachyte  occur  together  with  other  volcanic  rocks.  The  distinction  between  both  is  so  great  that  it  has 
scarcely  ever  faited  to  be  noticed  by  unbiased  observers  in  those  localities  where  both  rocks  occur.  To  unite  them  because 

(57) 


20  RICIITIIOFEN — NATURAL   SYSTEM 

/• 

Fam.  1st.  Sanidin-trachyte. — The  color  of  the  paste  varies,  but  it  usually  pre- 
sents light  shades  of  gray,  reddish,  and  reddish-brown  ;  its  texture  exhibits  all  the 
varieties  mentioned.  There  are  imbedded  in  it :  crystals  of  sanidin,  or  of  both  sani- 
din  and  oligoclase,  besides  mica  and  hornblende  ;  the  latter,  however,  is  frequently 
wanting.  To  this  family  belong  the  rocks  which  compose  the  trachytic  ranges  of 
Washoe  and  Esmeralda,  that  of  the  Drachenfels,  and  many  others. 

Fam.  2d.  Oligoclase-trachyte. — The  paste  is  of  the  same  color  as  in  the  rocks  of 
the  first  family,  though  darker  shades  prevail,  and  presents  a  similar  variety  of  text- 
ure. Imbedded  are,  chiefly,  crystals  of  oligoclase  and  hornblende,  the  former  being 
frequently  of  the  vitreous  variety  ;  the  latter  having  usually  the  shape  of  broad 
needles,  with  a  black  color  and  bright  cleavage-planes.  Besides  those  minerals, 
black  mica  is  of  frequent  occurrence.  The  rocks  of  this  family  are  ordinarily  associ- 
ated with  those  of  the  first  subdivision,  but  in  some  localities  are  not  accompanied  by 
them.  At  Lassen's  Peak  there  is  but  one  limited  space  where  rocks  of  both  families 
intermingle  ;  it  is  near  the  place  where  the  lava  has  been  ejected.  Apart  from  it,  the 
grand  currents  of  lava,  extending  to  from  ten  to  twelve  miles  distance  from  the  place 
of  ejection,  consist  merely  of  oligoclase-trachyte.  To  this  family  belong  those  varie- 
ties of  trachyte  which  were  called  "  domite  "  by  L.  v.  Buch. 

The  fact  that  the  occurrence  of  the  rocks  of  either  one  of  these  two  families 
will  frequently  exclude  those  of  the  other,  and  that,  even  in  those  localities  where 
they  are  associated  together,  they  will  occupy  separate  places  in  regard  to  geological 
superposition,  appears  to  indicate  that  the  distinction  of  these  two  subdivisions  forms 
an  approach  to  the  requirements  of  the  natural  system. 

ORDER  THIRD — PROPYLITE. 

The  rocks  of  this  order  have  hitherto  occupied  a  very  undecided  position  in 
the  different  classifications  of  rocks  proposed,  and  just  as  various  has  been  their  nomen- 
clature when  they  had  to  be  mentioned  in  geological  descriptions.  The  fact  that  they 
bear  close  resemblance  in  mineral  character  to  ancient  diorite,  while,  geologically,  they 
are  intimately  allied  to  volcanic  rocks,  has  been  the  principal  cause  of  this  uncertainty 
of  their  position.  In  Hungary  and  Transylvania,  they  occur  quite  extensively,  and, 
being  of  practical  importance  as  the  bearers  of  rich  metallic  veins,  have  had  to  be 
noticed  frequently  in  treatises  on  the  mines  of  those  countries.  Beudant  applied  for 
them  the  name  "  porphyric  greenstone,"  and  classified  them,  along  with  syenite,  among 

they  are  both  chiefly  composed  of  oligoclase  and  hornblende,  would  render  it  indeed,  practically,  a  very  difficult  and  compli- 
cated task  to  compare  the  geological  relations  of  different  volcanic  countries  on  the  strength  of  written  descriptions.  If  we 
now  follow  the  classification  of  J.  Roth,  we  find  the  application  of  the  name  trachyte  limited  to  those  volcanic  rocks  which 
contain  sanidin,  but  are  devoid  of  quartz.  All  those  numerous  varieties  which  do  not  contain  sanidin,  but  are  intimately 
allied  to  sanidin-tracliyte,  by  their  mode  of  geological  occurence  as  well  as  by  their  physical  characters,  are  excluded  from 
the  denomination,  and,  together  with  all  volcanic  rocks  composed  chiefly  of  hornblende  and  oligoclase,  are  united  into  one 
subdivision  of  andesite.  The  reasons  which  justify  the  separation  of  the  compounds  of  oligoelase  and  hornblende  into  three 
different  groups,  (oligoclase-trachyte,  hornblendia  propylite,  and  hornblendic  andesite)  will  be  detailed  in  the  following  pages. 
It  may  then  be  understood  why  the  adoption  of  the  different  systems  of  the  current  nomenclature  would  render  the  concise 
geological  description  of  volcanic  countries  extremely  difficult,  and  would  conceal  the  harmony  really  existing  in  the  rela- 
tions which  they  present  in  different  countries. 

(58) 


OF    VOLCANIC    ROCKS.  2t 

the  "transition  rocks."  Since  then,  the  names  greenstone,  greenstone-porphyry, 
diorite,  dioritic  porphyry,  and  others,  have  frequently  been  applied  for  the  same  rocks 
of  the  Carpathians.  Similar  names  have  been  used  for  them  when  they  were  mentioned 
as  occurring  in  other  countries,  as  for  instance  Mexico,  where  still  oftener  they  have 
been  simply  styled  "  porphyry."  In  1860,  having  had  sufficient  evidence  of  the  Ter- 
tiary age  of  these  rocks  and  their  close  connection  with  the  volcanic  rocks  of  that 
period,  I  separated  them,  in  a  treatise  on  some  volcanic  countries  in  Hungary,  already 
referred  to,  by  the  name  of  "greenstone-trachyte,"  from  the  remainder  of  those  rocks 
which  then  were  usually  comprehended  by  the  name  trachyte.  That  designation  has 
since  that  time  been  frequently  applied  in  geological  descriptions.  The  attempts  made 
to  classify  the  rocks  included  in  it  have,  however,  been  rather  unsuccessful.  J.  Roth 
combines  them  into  one  group  with  amphibol-andesite,  from  which  they  are  quite  dis- 
tinct as  regards  their  petrographical  as  well  as  their  geological  properties  ;  while  others 
considered  them  as  belonging  to  the  dioritic  rocks.  The  grounds  upon  which  they 
have  been  united  with  the  latter  are  purely  artificial,  since  diorite  is,  from  a  geological 
point  of  view,  widely  separated  from  "greenstone-trachyte."  Breithaupt  established 
a  new  name,  "  Timacite,"  for  a  variety  of  our  propylite,  which  is  of  very  limited  occur- 
rence, and  in  which  he  discovered  a  new  variety  of  hornblende,  called  by  him  "  gamsi- 
gradite."  Most  valuable  contributions  for  the  knowledge  of  our  "propylite,"  were 
recently  given  by  Dr.  Guido  Stache,  from  observations  made  in  Transylvania.  (Hauer 
and  Stache,  Geologic  Siebenbiirgens,  Vienna,  1863.)  He  discovered  the  occurrence, 
in  that  country,  of  quartz-bearing  varieties  in  greater  extent  than  they  have  been 
found  hitherto  at  any  other  place.  Stache  retains  the  name  "greenstone-trachyte," 
for  those  varieties  which  contain  no  quartz,  and  proposes  the  name  "  Dacite  "  (from 
the  Roman  province  of  Dacia  to  which  Transylvania  belonged)  for  those  of  which 
quartz  is  a  common  ingredient. 

These  statements  will  show  the  discrepancy  of  the  views  which  have  been 
entertained  in  regard  to  the  systematic  place  and  the  nomenclature  of  the  rocks 
under  consideration.  It  is  owing,  partly  to  their  twofold  affinity  with  other  rocks, 
(mineralogically  to  diorite,  and  geologically  to  volcanic  rocks)  and  partly  to  the  fact 
that  they  had,  until  lately,  not  been  made  the  object  of  study.  All  observations  made 
during  the  last  few  years  concur  in  this,  that  those  rocks,  wherever  they  have  been 
encountered,  form  a  distinct  link  in  the  range  of  Tertiary  and  Post-tertiary  eruptive 
rocks,  being  everywhere  the  first  of  them  in  age,  while  they  are,  in  regard  to  their 
mineral  character,  no  less  distinct  from  any  other  eruptive  rocks  originated  in  those 
periods.  They  constitute,  indeed,  a  more  natural  and  more  distinct  group  than  any 
of  the  other  volcanic  rocks,  and  it  has  become  desirable,  to  unite  them  under  a  com- 
mon designation.  As  no  prominent  property  distinguishes  them  from  diorite,  and  the 
derivation  of  the  name  from  one  certain  locality  did  not  appear  proper  for  the  desig- 
nation'of  rocks  of  wide  distribution,  geological  relations  alone  could  be  used  as  a  basis  for 
the  nomenclature.  The  rocks  under  consideration,  as  we  shall  hereafter  more  fully 
develop,  give  evidence,  in  all  localities  where  they  have  been  met  with,  of  having  re- 
opened the  eruptive  activity  after  ages  of  comparative  repose.  It  is  since  then  only, 

ii  (.59) 


22  RICIITIIOFEN NATURAL    SYSTEM 

that  this  activity  has  continued  with  extreme  violence  over  all  parts  of  the  globe, 
through  the  remaining  part  of  the  Tertiary  and  the  Post-tertiary  periods,  growing, 
however,  more  and  more  faint  during  the  latter.  Propylite  was,  in  fact,  the  precursor 
of  all  other  volcanic  rocks,  and  its  appearance  on  the  surface  inaugurated  a  grand 
revolutionary  activity  on  the  globe.  It  is  this  position,  at  the  entrance  as  it  were  to  a 
new  era  in  the  history  of  the  earth,  which  has  given  rise  to  the  name  "  propylite." 

Mode,  of  Geological  Occurrence. — The  position  of  propylite  at  the  bottom  of  all 
volcanic  rocks  is  its  most  important  geological  feature.  Its  age  has  in  no  instance  been 
ascertained  with  exactness.  The  nearest  approach  to  its  determination  was  made  in 
Northern  Transylvania,  where  I  found,  in  several  localities,  nuinmulitic  strata  inter- 
sected by  dykes  and  large  intrusive  masses  of  propylite,  and  covered  by  accumulations 
of  it.  As  the  greater  part  of  the  volcanic  rocks  in  that  country  are  of  Miocene  age, 
the  ejection  of  propylite  must  have  taken  place  either  in  the  latter  part  of  the  Eocene 
or  in  the  earlier  part  of  the  Miocene  epoch.  In  view  of  the  facts,  that  volcanic  rocks 
have  nowhere  been  observed  to  be  anterior  to  the  Eocene  (probably  even  not  prior  to 
the  Miocene)  epoch,  and  that  propylite  is  always  allied  to  them  as  the  first  link  in  the 
order  of  succession,  it  may  be  inferred  that  propylite  is,  in  general,  of  Tertiary  age, 
until  proofs  to  the  contrary  may  be  found. 

The  forms  of  propylitic  mountains  can  be  observed  only  in  rare  instances,  since 
they  are  usually  covered  by  other  volcanic  rocks,  especially  by  andesite.  This  circum- 
stance may  also  explain  the  fact  of  the  comparatively  rare  occurrence  of  propylite  as  a 
surface-rock.  In  the  environs  of  Bisztritz,  in  Northern  Transylvania,  it  forms  several 
high,  isolated  cones  with  steep  slopes,  resting  on  Eocene  strata ;  their  fine,  dome- 
shaped  appearance  is  scarcely  surpassed  in  beauty  by  that  of  any  other  kind  of  rock. 
No  estimate  can  be  made  in  regard  to  the  relative  bulk  of  propylite  which  has  been 
ejected,  on  account  of  its  being  overlaid  by  andesite  and  trachyte.  It  occurs,  as  far 
as  exposed  to  view,  in  quite  considerable  accumulations,  at  Nagybanya  and  Kapnik, 
in  Hungary,  in  Washoe,  and  at  Silver  Mountain,  and  covers  large  areas  in  Mexico, 
where  it  overlies  the  Cretaceous.  It  appears  to  have  been  profusely  ejected  through 
fissures,  and  its  emission  not  to  have  been  accompanied  by  volcanic  action  proper  ;  no 
distinct  traces,  at  least,  have  been  found  of  propylitic  volcanoes.  Massive  eruptions 
are  known  to  have  occurred,  besides  the  places  mentioned,  in  several  other  parts  of 
the  southern  slope  of  the  Carpathians,  on  the  highlands  of  Armenia,  and  in  Mexico, 
while  the  occurrence  of  propylite  in  Bolivia,  the  Altai  Mountains,  Northern  China, 
and  some  other  countries,  may  be  inferred  from  the  description  of  their  geology. 

Propylite  has  been  repeatedly  considered  to  be  a  sedimentary  rock,  metamor- 
phosed in  situ.  The  cause  of  this  opinion  was  probably  the  fact  that  there  are  rocks 
which  undoubtedly  have  had  that  mode  of  origin,  and  share  with  propylite  the  resem- 
blance in  mineral  character  to  diorite.  The  eruptive  origin  of  propylite  is,  however, 
evident,  as  it  intersects  stratified  rocks  in  the  shape  of  dykes.  In  Washoe  and  Silver 
Mountain,  moreover,  there  are  extensive  accumulations  of  propylitic  breccia  and 
stratified  tufa.  The  latter  consists  of  alternate  layers  of  coarse  conglomerate,  fine- 

(60) 


OF   VOLCANIC   ROCKS.  23 

grained  propylitic  detritus  and  massive  propylite,  and  the  entire  system  is  intersected 
by  dykes  of  the  latter. 

A  particular  interest,  which  has  a  practical  bearing  of  some  importance, 
attaches  to  propylite.  inasmuch  as,  notwithstanding  its  limited  occurrence,  it  yields 
probably  a  larger  amount  of  silver  than  any  other  rock.  Several  of  the  principal 
silver-bearing  veins,  as  the  Comstock  vein  in  Washoe,  the  celebrated  veins  in  Hungary 
and  Transylvania,  as  well  as  some  of  those  in  Mexico,  and  probably  too  in  Bolivia,  are 
enclosed  in  propylite. 

Mineral  Composition. — In  propylite  are  united  the  petrographical  properties  of 
ancient  dioritic  rocks  with  those  of  andesite  and  oligoclase-trachyte.  Varieties  are 
numerous,  but  produced  more  frequently  by  the  difference  of  the  component  minerals  in 
size  and  relative  proportion  than  by  the  texture,  which  remains  porphyritic  in  every  in- 
stance. The  most  common  varieties  consist  of  a  fine-grained,  microcrystalliue  paste  of 
dark  green  or  greenish  brown,  more  rarely  of  reddish  and  dark  gray  colors,  in  which 
are  imbedded  crystals  of  oligoclase  and  hornblende  ;  the  former  is  of  whitish  or  light 
green  color,  the  latter  ordinarily  dark  green  and  fibrous,  seldom  black  with  bright 
cleavage-planes  (as  is  the  case  with  gamsigradite).  The  paste  appears  to  be  a  fine- 
grained aggregation  of  the  same  two  minerals  (the  feldspathic  ingredient  prevailing), 
with  the  admixture  of  titanic  iron,  and  to  owe  its  ordinarily  green  color  to  the  profuse 
dissemination  of  small  particles  of  green,  fibrous  hornblende.  Sporadic  crystals  of 
augite  are  occasionally  met  with  in  some  of  the  common  varieties,  while  others  contain 
rounded  grains  of  quartz  sparsely  enclosed.  Recent  observations  have  led  to  the  dis- 
covery of  two  series  of  varieties  deviating  from  the  usual  composition  as  explained. 
One  of  them  is  produced  by  the  increasing  proportion  of  the  grains  of  quartz  ;  the 
other  by  the  more  profuse  occurrence  of  augite.  The  former  was  found  by  Stache,  in 
Western  Transylvania  ;  the  latter  observed  by  me  at  Silver  Mountain.  Both  appear 
to  be  of  a  limited  geographical  distribution,  but  are  of  great  interest,  as  they  extend 
the  limits  of  the  order  of  propylitic  rocks,  without  rendering  either  their  petro- 
graphical character,  or  their  geological  relations  less  distinct.  In  regard  to  the  latter, 
it  may  be  stated  that  the  entire  range  of  the  quartzose  to  augitic  varieties  was  anterior 
in  age  to  andesite. 

Difference  of  Propylite  from  other  Volcanic,  Rocks  nearly  related  to  it. — The  rocks  of 
two  other  orders  have  an  affinity  to  propylite,  namely  oligoclase-trachyte  and  ande- 
site. The  principal  ingredients  of  either  are  hornblende  and  oligoclase,  and  either  of 
them  contains  crystals  of  both  these  minerals  imbedded  in  a  paste.  Yet  there  is  a 
notable  difference  in  character  between  these  two,  as  well  as  between  either  of  them 
and  propylite.  It  is  so  conspicuous  to  the  eye  that  the  respective  rocks  have  ordi- 
narily been  distinguished  in  geological  descriptions,  even  in  those  of  older  time  ;  and 
even  the  unprofessional  eye  would  be  able  to  distinguish  the  three  groups  in  a  collec- 
tion of  specimens  belonging  to  them.  Yet,  it  escapes  description.  It  may,  at  this 
present  time,  safely  be  founded  on  what  the  botanist  would  call  "  habitus,"  a  certain 
general  character  which  it  is  as  easy  to  recognize  by  the  eye  as  it  is  difficult  to  describe 
it  in  words,  and  impossible  to  define  its  causes.  It  is  probable  that  observations,-  such 

(61) 


24  RICIITHOFEN NATURAL    SYSTEM 

as  Sorby  has  made  in  reference  to  those  minute  differences  of  texture,  which  can  only 
be  detected  with  the  aid  of  the  microscope,  and  H.  Rose  in  regard  to  the  modifications 
of  silica  and  their  causes,  aided  by  exact  chemical  analysis  and  experiments  made 
with  the  view  of  enquiring  into  the  differences  of  origin  of  such  eruptive  rocks  as 
differ  from  each  other  in  texture,  will,  if  further  prosecuted,  reveal  the  true  nature 
and  cause  of  the  properties  which  distinguish  the  rocks  of  these  three  different 
orders. 

A  few  of  the  more  palpable  differences  may  here  be  noticed.  Propylite  is 
essentially  of  greenish  color,  and  some  of  its  varieties  resemble  diorite  in  composition 
and  texture;  andesite  is  of  blackish  color  and  approaches  basalt  in  aspect;  trachyte  is 
of  various  colors  and  shades,  among  which  green  and  black  are  rarest  of  occurrence, 
and  in  regard  to  its  external  characters  resembles  rhyolite  more  than  any  other  rock. 
Oligoclase,  in  trachyte,  is  frequently  of  the  vitreous  variety,  scarcely  ever  so  in  pro- 
pylite  and  andesite.  Hornblende  is  an  essential  ingredient  in  these  two,  not  so  in  the 
former ;  it  is  of  fibrous  texture  and  green  color  in  most  varieties  of  propylite,  black  in 
andesite  and  trachyte.  Mica  is  seldom  wanting  in  the  latter,  while  it  is  not  of  com- 
mon occurrence  in  propylite  and  andesite.  Titanic  iron  enters  largely  into  the  compo- 
sition of  these,  and  is  contained  in  smaller  proportion  in  trachytic  rocks.  The  latter 
excel  by  having  the  greatest  variety  of  texture,  while  propylite  has  among  the 
three  the  most  perfect  porphyritic  texture,  which  has  given  rise  to  its  frequent 
popular  designation  "  porphyry  ;"  this  name  has  never  been  applied  to  andesite  or 
trachyte. 

The  enumeration  of  all  these  trifling  differences  is,  however,  insufficient  to 
express  the  marked  distinction  which  exists  in  the  external  characters  of  propylite, 
trachyte  and  andesite.  As  a  similar,  and  even  more  conspicuous,  distinction  manifests 
itself  in  their  geological  relations,  we  have  to  consider  the  existe  .ce  of  those  three 
natural  orders  as  a  fact  founded  on  observations,  although  we  may  be  utterly  unable  . 
to  explain,  and  even  to  express  it  in  words. 

Subdivisions. — The  remarks  made  in  regard  to  the  mineral  composition  of  pro- 
pylite have  shown  that  the  range  of  its  varieties  may  conveniently,  and  in  harmony 
with  geological  occurrence,  be  subdivided  into  three  parts  : 

Farn.  1st.  Quartzose  Propyltte  or  Dacite. — This  embraces  rocks  which,  though 
having  in  general  a  similar  composition  to  those  of  the  following  family,  contain  be- 
sides, rounded  grains  of  quartz,  sometimes  in  considerable  proportion.  They  occur  in 
the  western  part  of  Transylvania,  where  their  outbreaks  succeeded  those  of  rocks  of 
the  second  family,  and  preceded  those  of  andesite.9  Similar  rocks  have  been  observed 
in  Sinaloa  (Mexico). 

Fam.  2<?.  Hbrnblendic  Propylite. — Rocks  composed  chiefly  of  hornblende  and 
oligoclase,  as  described  above.  This  family  embraces  vastly  the  majority  of  all  pro- 
pylitic  rocks  observed,  among  others  those  of  Washoe.  Breithaupt's  "  tirnacite  " 
is  one  of  its  varieties. 

— — — . ; 

9   Fully  described  by  G.  Stachc,  loc.  cit. 
(62) 


OF   VOLCANIC    ROCKS.  25 

Fam.  %d.  Augitic  Propylite. — Rocks  distinguished  by  the  accession  of  augite 
among  the  ingredients  of  the  rocks  of  the  foregoing  family.  It  is  present  in  greater 
or  less  quantity,  sometimes  predominating  over  the  hornblende.  To  this  family  be- 
longs the  propylite  of  Silver  Mountain,  which  contains  augite  in  larger  proportion 
than  any  other  known  variety. 

ORDER  FOURTH — ANDESITE. 

The  history  of  the  name  "  andesite  "  has  been  noticed  conjointly  with  that  of 
"  trachyte."  At  the  time  when  the  former  name  was  first  proposed  for  certain  rocks 
of  the  Andes,  a  few  specimens  of  which  had  been  brought  to  Europe,  it  was  intended 
as  a  designation  of  an  accidental  variety,  proper  to  that  mountain  range.  But  those 
same  specimens  have  proved  since  to  be  the  type  of  one  of  the  most  important  groups 
of  volcanic  rocks,  which  is  distinct  from  others  in  character,  and  has  a  wide  distribu- 
tion. 

Mode  of  Geological,  Occurrence. — Andesite  vies  with  basalt  in  regard  to  the 
quantity  of  matter  ejected  to  the  surface,  and  probably  excels  the  same  in  this  respect. 
In  most  of  those  parts  of  the  Andes,  in  regard  to  the  geology  of  which  we  possess 
reliable  information,  it  foi'rns  the  chief  bulk  among  volcanic  rocks.  The  same  is  the 
case  on  the  southern  slopes  of  the  Carpathians,  at  Nangasaki  in  Japan,  and  on  the 
islands  of  Luzon  and  Java.  Andesite  succeeded  the  ejection  of  propylite,  and  pre- 
pared the  way  to  that  of  trachyte.  Preeminently  the  greater  part  of  it  has,  to  all 
appearance,  been  ejected  through  extensive  fissures  ;  that  is,  it  has  been  produced  by 
what  we  styled  massive  eruptions  ;  though  andesitic  volcanoes,  too,  are  not  of  rare 
occurrence,  and,  including  those  which  are  extinct,  appear  to  have  been  particularly 
grand  in  their  activity.  It  may  be  supposed  that  many  of  the  former  craters  have  been 
destroyed.  Andesitic  mountains  are  characterized  by  monotony  in  scenery.  Thev 
form  continuous  ranges,  which  are  often  of  considerable  elevation  and  extent,  but  ex- 
hibit gentle  outlines  in  their  summits  as  well  as  in  their  slopes.  Breccias  only,  which 
accompany  the  solid  rock  ordinarily  in  vast  quantities,  cause  local  interruptions  of  the 
monotony  by  their  more  rugged  forms.  They  appear  in  castle-shaped  rocks  on  the 
crests  of  andesitic  mountains,  and  form  high  walls,  naked  and  steep,  along  their  slopes. 
Being  more  liable  to  destruction  by  the  erosive  action  of  water  than  solid  andesite, 
they  frequently  compose  the  sides  of  steep  ravines  and  canons. 

Mineral  Composition. — Andesite  is  always  of  dark  color,  mostly  blackish, 
though  frequently  reddish-brown  on  the  weathered  surface.  Its  mineral  composition 
varies,  though  it  is  confined  within  more  narrow  limits  than  that  of  propylite.  We 
may  distinguish,  in  regard  to  it,  two 

Subdivisions,  which  are  connected  by  gradual  passage  in  composition  and,  in 
a  greater  measure,  by  geological  relations. 

Fam.  1.  ffornblendic  Andesite. — Paste  of  bluish-black  to  dark-gray  color,  and 
of  microcrystalline»texture,  which  passes  by  gradual  steps  into  that  of  obsidian.  In 
the  former  case  it  is  frequently  vesicular  like  trachyte.  There  are  imbedded  in  it 

(63) 


26  RICHTHOFEN — NATURAL   SYSTEM 

small  but  distinct  tabular  crystals  of  oligoclase,  fine  grains  of  titanic  iron,  small, 
elongated  columns  of  hornblende,  sometimes  mica,  and  in  most  varieties  a  few  isolated 
crystals  or  rounded  grains  of  augite.  Frequently  these  different  crystals  are  so  small 
as  to  be  no  longer  recognizable,  except  by  the  aid  of  the  microscope.  The  rock  is 
then  similar  in  character  to  certain  varieties  of  melaphyr,  but  may  be  distinguished 
by  its  vesicular  texture,  which  is  scarcely  ever  wanting  in  the  hornblendic  varieties. 
This  family  comprises  vastly  the  greater  portion  of  all  andesitic  rocks. 

Fam.  2.  Avgitic  AndesHe. — Two  groups  of  varieties  may  be  distinguished 
among  the  rocks  of  this  family.  Those  of  the  first  have  a  paste  of  an  oil-brown  color 
passing  into  black,  a  compact  appearance,  and  ordinarily  a  microcrystalline  tex- 
ture which  passes  into  that  of  porcelain.  Vesicular  inflation  does  ordinarily  not 
occur.  Crystals  of  a  monoclinohedric  feldspar  (probably  labrador)  are  almost  invari- 
ably enclosed,  and  frequently  accompanied  by  hornblende  and  augite.  For  the  rocks 
of  this  group  the  name  "  trachydolerite  "  has  particularly  been  applied.  The  other 
group  comprises  certain  varieties  for  which  the  name  "  anamesile  "  has  not  rarely  been 
used,  and  which  are  of  dark-gray  colors.  Their  texture  is  in  most  cases  vesicular. 
The  enclosed  minerals  are  the  same  as  with  the  other  group  ;  but,  while  for  this 
the  feldspathic  ingredient  is  more  characteristic,  and  frequently  alone  present  in  large 
crystals,  the  rocks  of  the  second  group  are  distinguished  by  the  predominance  of  well 
formed  crystals  of  augite  and  hornblende,  while  those  of  feldspar  are  often  so  small 
that  they  cannot  be  distinguished  by  the  eye.  The  rocks  of  both  these  groups  contain 
titanic  iron  in  larger  quantity  than  those  of  the  first  family,  and  are  of  greater  specific 
gravity.  Olivine  enters  occasionally  into  their  composition,  in  very  subordinate  quan- 
tity. Geologically,  the  rocks  of  this  family  are  closely  allied  to  hornblendic  audesite. 
They  have  succeeded  it  in  age,  and  are  limited  in  their  geographical  distribution  to  those 
places  where  hornblendic  varieties  had  been  ejected  before,  frequently  intersecting  and 
overlying  them.  Notwithstanding  the  resemblance  which  they  bear  to  certain  varie- 
ties of  basaltic  rocks,  they  appear  to  be  never  associated  with  them  geologically. 

ORDER  FIFTH — BASALT. 

No  one  of  the  names  applied  to  volcanic  rocks  is  of  equal  antiquity  with  that  of 
"  basalt,"  and  with  none  there  has  ever  been  less  change  of  opinion,  and  uncertainty  in 
respect  to  its  application.  It  may  be  inferred  from  this  fact  that  the  rocks  comprised 
by  that  name  are  very  distinct  in  character  and  little  liable  to  variation,  which  is  indeed 
true  for  the  typical  rocks  of  the  order.  But  there  have  to  be  associated  in  the  same 
order  with  them  certain  other  rocks,  which,  though  nearly  related  to  basalt  in  regard  to 
their  chemical  and  mineral  composition,  differ  from  it  in  the  more  conspicuous  external 
characters. 

Mode  of  Geological  Occurrence. — Basaltic  rocks  are  more  independent  than  those 
of  any  of  the  foregoing  orders  (perhaps  with  the  exception  of  propylite),  in  respect 
to  their  epoch  of  ejection  as  well  as  to  their  geographical  distribution.  In  the  order 
of  time  they  succeeded  next  to  rhyolite,  but  locally,  both  are  tisually  separated. 
Basalt  occurs  always  in  the  neighborhood  of  more  ancient  volcanic  rocks,  but  the  cases 
(64) 


OF   VOLCANIC    ROCKS.  27 

are  rare  when  it  intersects  them  and  expands  over  them.  Almost  invariably  it  is  found 
accompanying  their  ranges  at  some  distance,  forming  itself  extensive  ranges  ;  more 
frequently,  however,  it  occurs  in  groups  or  lines  of  isolated  outliers.  It  may  some- 
times appear  to  have  no  connection  with  the  distribution  of  older  volcanic  rocks.  But 
closer  investigation  will  always  show  it  to  be  within  or  in  the  neighborhood  of  the 
limits  of  former  eruptive  activity.  The  belts  along  which  this  had  taken  place  may 
be  locally  interrupted  for  quite  a  distance,  and  the  gap  be  filled  by  some  isolated  out- 
breaks of  basalt,  or  the  latter  may  extend  the  limits  of  those  belts,  in  length  as  well 
as  in  width.  It  is  a  strange  phenomenon  that  these  isolated  outbreaks  of  basalt  occur 
particularly  in  connection  with  granite.  It  intersects  this  rock  very  frequently  in  small 
dykes,  and  expands  over  it  in  thin  sheets.  This  connection  of  granite  and  basalt  is 
very  conspicuous  on  the  eastern  slope  of  the  Sierra  Nevada,  and  along  the  belt  of 
basaltic  eruptions  which  traverses  the  middle  part  of  Germany.  Such  places  are  often 
the  seat  of  basaltic  volcanoes,  which,  though  now  mostly  extinct,  are  also- otherwise  of 
very  frequent  occurrence. 

Mineral  Composition. — Basalt  is  the  representative  of  what  Bunsen  has  called 
the  "  normal  pyroxenic  type,"  and  is  thereby,  as  well  as  by  its  specific  gravity,  mineral 
composition,  and  little  variety  of  texture,  the  reverse  of  rhyolite,  which  represents  the 
"normal  trachytic  type."  All  the  essential  ingredients  of  basalt,  and  most  of  the 
minerals  which  enter  accidentally  into  its  composition,  are  different  from  those  which 
are  characteristic  of  rhyolite.  Augite,  labrador  and  titanic  iron,  imbedded  in  a  paste 
consisting  essentially  of  the  same  minerals,  constitute  generally  the  rocks  of  this  order  ; 
or  labrador  may  be  replaced  by  leucite,  nepheline  or  a  zeolitic  substance  ;  or  other  min- 
erals may  enter  into  the  composition,  such  as  olivine,  basaltic  hornblende,  hauyne, 
apatite  and  black  mica  ;  and  in  many  instances  basaltic  rocks  consist  merely  of  a 
fine-grained  aggregation  of  the  different  minerals  mentioned.  Ordinarily,  the  paste 
has  a  microcrystalline  texture,  and  may  either  contain  crystallized  minerals  imbedded, 
or  may  be  devoid  of  them.  This  mode  of  texture  passes  by  gradual  steps  into  that  of 
obsidian,  which  is  the  only  hyaline  variety  occurring.  Among  the  peculiarities  of  these 
rocks,  and  which  may  also  in  some  measure  be  observed  with  augitic  andesite,  is  the 
occurrence  of  rounded  cavities  filling  the  rock  and  giving  it  a  cellu^ir,  sometimes 
spongy  appearance.  It  is  different  in  nature  from  pumice-stone,  which  is  a  modifica- 
tion of  hyaline  texture,  while  those  cavities  occur  in  microcrystalline  rocks. 

Subdivisions. — Fam.  1st.  Dolerite  (including  nepheline-dolerite  and  the  greater 
part  of  anamesite). — A  crystalline  aggregation  of  augite  and  labrador  with  titanic  iron. 
Labrador  is  usually  replaced  by  nepheline,  either  in  part  or  totally.  Accessory  min- 
erals are  :  olivine,  hornblende,  apatite,  black  mica. 

Fam.  2(7.  Basalt. — Paste  of  dark-gray  or  black  color,  and  of  the  varieties  of  text- 
ure mentioned  ;  it  constitutes  either  the  mass  of  the  rock  by  itself,  or  encloses  olivine, 
augite  and  labrador,  in  crystals  or  crystalline  grains.  Besides  these  are  frequently 
enclosed :  titanic  iron,  black  mica,  rubellan,  zircon,  apatite  and  other  minerals.  This 
family  comprises  nearly  the  whole  bulk  of  the  rocks  which  constitute  the  basaltic 
order,  the  two  other  families  being  of  rare  occurrence. 

(65) 


28  RICHTHOFEN — NATURAL   SYSTEM 

Fam.  3d.  Leucitophyre. — Rocks  of  porphyritic  texture,  crystals  of  augite  and 
leucite  being  imbedded  in  the  paste. 

It  will  be  noticed  that  no  place  has  been  assigned  in  this  classification  to  phon- 
olite. As  regards  those  rocks  for  which  this  name  was  first  proposed,  onr  knowledge 
is  still  quite  limited.  It  appears  that,  notwithstanding  their  comparatively  rare  occur- 
rence, they  form  a  distinct  natural  group  closely  allied  to  basalt ;  but  they  are  so  dif- 
ferent from  true  basalt  as  regards  lithological  characters,  that  they  should  not  be 
classified  with  it  before  further  observations  will  have  determined  their  real  position 
in  reference  to  the  natural  families  of  volcanic  rocks.  The  name  phonolite  has,  how- 
ever, been  so  much  extended  in  its  application,  that  this  task  is  not  so  easy  to  accom- 
plish. Some  external  properties,  easy  of  recognition  to  superficial  observation,  such 
as  a  certain  tabular  structure,  lithoid  texture  of  the  paste,  with  small,  bright  crystals 
of  feldspar  enclosed,  and  the  peculiarity  of  ringing  by  a  blow  of  the  hammer,  which 
have  been  often  considered  as  the  characteristic  features  of  phonolite,  are  just  as  com- 
mon with  certain  varieties  of  trachyte  and  rhyolite,  and  even  with  some  of  propylite. 
It  must  be  ascribed  to  this  reason,  that  the  name  phonolite  has  been  used  for  the 
designation  of  rocks  which  bear  an  accidental  resemblance  to  true  phonolite,  but  are 
distinct  from  it  in  nature.  It  has  probably  been  oftener  applied  to  rocks  belonging  to 
the  trachytic  order  than  to  such  as  have  the  distinguishing  features  of  those  varieties 

for  which  the  name  has  been  first  used. 

• 

CORRELATION  OF  THE  FIVE  ORDERS  OF  VOLCANIC  ROCKS. 

In  the  foregoing  pages  I  have  attempted  to  lay  down  the  outlines  of  a  classifi- 
cation of  the  volcanic  rocks  by  natural  principles,  and  to  apply  a  nomenclature  which 
should  be  appropriate  to  these,  and  embrace,  at  the  same  time,  the  most  current  of 
existing  names.  It  is  the  next  object  of  this  paper  to  prove  that  these  rocks  are 
mutually  connected  by  definite  relations,  and  that  their  totality,  in  virtue  of  this 
property,  forms  actually  what  maybe  called  a  system  in  nature,  and  that  the  form  into 
which  we  have  tried  to  bring  it,  imperfect  though  it  must  be,  is  an  approach  towards 
its  expression.  In  order,  therefore,  to  fully  realize  the  philosophy  of  the  natural 
system,  we  have  to  contemplate  the  relations  which,  firstly,  the  rocks  of  the  different 
orders  offer  mutually  among  themselves,  and  by  which,  secondly,  they  are  connected 
as  an  entire  class  with  ancient  eruptive  rocks  ;  while  we  will  have,  thirdly,  to  examine 
into  the  mode  of  origin  of  volcanic,  and  of  eruptive  rocks  in  general,  in  order  to 
establish  the  nature  of  their  fundamental  difference  from  sedimentary  and  metamor- 
phic  rocks.  Our  task  is  thus  three-fold.  The  present  chapter  will  be  devoted  to  the 
first  order  of  relations.  They  may  be  considered  from  several  points  of  view,  the 
more  important  of  which  are  :  chemical  and  mineral  composition,  geographical  distri- 
bution, and  all  those  complex  relations  which  may  be  comprehended  in  the  term 
"mode  of  geological  occurrence."  We  will  confine  ourselves  to  the  last  point  of  view. 
But  even  with  this  restriction,  we  can  only  trace  general  outlines. 

(fiC) 


OF   VOLCANIC    ROCKS.  29 

Laws  relative  to  the  Age  of  Massive  Eruptions. 
i 

The  succession  of  massive  eruptions  during  the  Tertiary  and  Post-tertiary  ages 
has  taken  place  in  the  following  order  : 

1st.  Propylite. 

2d.  Andesite. 

3d.  Trachyte. 

4th.  Rhyolite. 

5th.  Basalt. 

This  singular  mode  of  succession,  in  which  no  regularity  (as  to  increase  or 
decrease  of  silica  or  specific  gravity,  or  as  to  a  gradual  change  of  mineral  composition) 
can  be  discovered  at  first  sight,  and  which  might  indeed  appear  to  be  devoid  of  order, 
and  to  bear  the  character  of  such  a  succession  as  might  have  been  occasioned  by  the 
cooperation  of  accidental  circumstances  in  one  single  country,  can  nevertheless  be 
proved  to  exist  in  widely  separated  parts  of  the  globe.  It  may  justly  be  objected  to 
this  assertion,  that  observations  in  regard  to  the  relative  age  of  different  volcanic  rocks 
are  scarce,  and  hardly  sufficient  to  establish  definitely  such  a  law.  But  hitherto  no 
deviation  from  it  has  been  discovered,10  and  it  appears  to  be  true  for  all  volcanic  regions 
on  the  globe,  though  with  this  restriction,  that  the  epochs  marked  in  each  country  by 
the  ejection  of  rocks  of  any  certain  orders  have  not  been  contemporaneous  in  different 
countries.  The  commencement  of  eruptive  activity  in  the  Tertiary  epoch  has  been  ear- 
lier at  one  place  than  at  another  ;  it  is  its  further  mode  of  development  in  regard  to  the 
nature  of  the  matter  ejected  which  has  everywhere  been  regulated  by  the  same  definite 
relations,  though  it  has  been  independent,  in  some  measure,  at  each  place,  or,  to  use 
a  more  correct  expression,  over  the  area  of  each  belt  of  eruptive  activity.  We  have 
to  mention  another  restriction.  Abrupt  passage  may  be  said  to  be  an  almost  unknown 
conception  in  geological  matters,  where  the  order  in  time  is  concerned  ;  nor  has  it  to 
be  applied  to  the  order  of  succession  of  volcanic  rocks.  The  eruptions  of  propylite 
appear  not  to  have  been  interrupted  by  the  ejection  of  any  other  rocks.  This  may 
too  be  said,  though  less  strictly,  of  the  andesitic  epoch.  But  some,  as  it  were,  retarded 
eruptions  of  andesite,  which,  however,  have  always  been  insignificant,  may  occasion- 
ally be  traced  during  the  first  part  of  the  trachytic  epoch,  and  similar  relations  exist 
between  trachyte  and  rhyolite.  Basalt,  however,  appears  to  have  had  everywhere  its 
own  epoch  of  ejection,  uninterrupted  by  the  massive  eruptions  of  any  other  rock  of 
the  orders  just  mentioned. 

We  proceed  to  a  short  review  of  some  salient  facts  observed  in  those  coun- 
tries where  the  mutual  relations  of  volcanic  rocks  in  regard  to  their  age  have  been 
made  an  object  of  study. 

In  Hungary  and  Transylvania,  propylite,  as  we  have  had  occasion  to  mention, 
was  ejected  first  of  all  volcanic  rocks,  and  may  be  said  to  have  inaugurated  all  sub- 

10  It  must  be  borne  in  mind  that  we  are  speaking  of  massive  eruptions.     Apparent  exceptions  are  known  to  occur 
with  rocks  ejected  from  volcanoes,  of  which  mention  will  be  made  hereafter. 

i  (67) 


30  KICIITHOIEN NATl'KAI-    SYSTKM 

sequent    eruptions    of  those  belonging  to    other   orders.      At   several  places  it  has 
been  observed  to  intersect  Eocene  strata,  and  to  expand  above  them.      The  coun- 
try of  Nagybanya,  Felsobanya  and  Kapnik,  the    celebrated  silver-bearing  veins  of 
which  are  enclosed  in  propylite,  offers  especially  conspicuous  illustrations  ;  not  less, 
from  the   descriptions  given   by  G.  Staclie,  the  "  Erzgebirge  "  of  Transylvania,  where 
the  same  kind  of  rock  is  rich  in  mineral  veins.     In  this  country,  hornblendic  propylite 
forms  an   older  series,  followed  by  eruptions  of  highly  quartziferous  varieties.      In 
both   countries,  but  chiefly  in  that  first  named,  and  at  several  other  places  along  the 
southern  slope  of  the  Carpathians,  andesite  maybe  seen  intersecting  propylite  in  large 
massive  dykes,  and   towering  up  above   it  in  mountain  ranges.     Andesite  composes 
entirely  the   Hargitta-range,  which  extends  over  one    hundred    miles  in  length,  and 
twenty-five  in  width  ;    the  Vihorlat-Gutin-range,  which  is  of  still   larger  dimensions, 
and  the  Eperies-Kaschau-rangc  ;  all  of  -which  are  densely  wooded,  and  of  a  gloomy, 
monotonous  aspect.     The  only  change  observable  on  their  summit  ranges  is  a  more  or 
less  dark  color  of  the  rock,  occasioned  by  the  predominance  of  augite  or  hornblende  in 
its  composition,  ((he  augitic  varieties  being  invariably  of  more  recent  age  than  the 
hornblendic),  while  their  slopes,  and   particularly  their  ends,  present  a  much  greater 
variety  in  rocks  as  well  as  in  scenery.     It  is  here  that  the  more  silicious  volcanic  rocks 
are  encountered.     Trachyte  is  of  rare  occurrence  ;  but  it  forms  several  isolated  cones, 
some  of  which,  on  account  of  their  prominent  position,  were  crowned  by  castles  in  the 
middle    ages.     Rhyolite    is  much  more  frequently   met    with,  bursting  forth  on    the 
flanks,  and  skirting  the   foot  of  the  andesitic  ranges,  particularly  where  they  verge 
towards  the  Hungarian  plains,  which  in  the  rhyolitic  epoch  were  still  covered   by  a 
shallow  and  slowly  retiring  sea.     It  projects  against  this  in  promontories,  which  are 
now  covered   by  the    most   celebrated  of  the  Hungarian  vineyards,  those  of  Tokay 
among  others.     The  boundaries  of  these  vineyards  towards  the  adjoining  beech  forests 
mark  the  dividing  line  between  rhyolite  and  andesite.     An  interesting  mode  of  occur- 
rence of  the  former  may  be  witnessed  in  large  circular  or  amphitheatrical  basins  which 
are  surrounded  by  andesite.     Such  places  are  the  theater,  especially,  of  the  volcanic 
activity  connected  with  the  outbreak  of  rhyolite,  and  abound  in  endless  hyaline  vari- 
eties of  the  same.     Telkibanya  is   the  most  interesting  among    the    localities  of  this 
description.     There  is  no  lack  of  evidence  to  prove  that  trachyte  and  rhyolite  are  both 
of  more  recent  age  than  andesite,  while  the  assertion  that  trachyte  preceded  rhyolite 
in  age,  rests  only  on  a  few  though  conclusive  observations.     Basalt  occupies  a  singular 
position  in  the  geology  of  Hungary.  '  It  keeps  altogether  aloof  from  the  places  occupied 
by  the  other  four  orders  of  volcanic  rocks,  and  forms  extensive,  though  isolated,  hills  at 
some  distance  from  them,  scattered  over  a  wide  range  of  country.     It  would  be  difficult 
to  determine  its  relative  age,  but  for  the  volcanic  sediments  which  were  of  formation 
contemporaneous  with  the  ejection  of  the  different  volcanic  rocks,  and  have  been  spread 
over  wide  areas  at  the  bottom  of  the  then  existing  sea,  with  fossils  occasionally  imbedded. 
In  several  localities,  especially  in  the  neighborhood  of  Kaschau,  basaltic  sediments  may 
be  seen,  covering  those  composed  of  rhyolitic  matter,  while  at  Gleichenberg  in  Styria, 
Mr.  Franz  von  Haucr  has  observed  fragments  of  rhyolite  enclosed  in  basalt. 

The  observation  of  these  relations  in  Hungary  and  Transylvania  has  first  given 

(08) 


OF   VOLCANIC    ROCKS.  31 

rise  to  the  establishment  of  the  above  mentioned  law  of  the  periodical  succession  of 
volcanic  rocks.  n  It  has  since  been  corroborated  by  observations  made  in  other  coun- 
tries far  remote  from  the  Carpathians.  Near  Nangasaki,  in  Japan,  andesite  preceded 
trachyte  in  age,  and  of  the  former  there  are  two  varieties  represented,  one  augitic 
and  one  hornblendic,  of  which  the  former  is  of  more  recent  origin  than  the  latter  : 
thus,  even  the  more  minute  relations  observed  in  the  Carpathians  are  repeated  in 
other  countries.  The  islands  of  Java  and  Luzon  are  too  intensely  volcanic  and 
covered  with  lava  to  aid  in  establishing  the  laws  in  respect  to  massive  eruptions 
without  very  close  observation.  But  another  region  offering  copious  evidence  is  that 
of  the  Sierra  Nevada,  together  with  the  adjoining  parts  of  the  Great  Basin.  Observa- 
tions in  these  countries  are  still  limited  as  regards  our  present  subject.  The  great 
part,  however,  which  volcanic  rocks  take  in  their  composition,  as  well  as  in  that  of  the 
highlands  of  Mexico,  and  of  the  entire  range  of  the  Andes,  promises  to  make  the 
Western  Coast  of  America  the  most  prolific  source  of  observations  necessary  for  the 
definite  establishment  of  geological  laws  of  which,  at  the  present  day,  we  can  only 
trace  the  first  foundation. 

In  Washoe,  a  country  adjoining  the  Sierra  Nevada  immediately  to  the  east,  propy- 
lite  forms  extensively  the  foundation  for  all  other  volcanic  rocks,  which  fact  proves 
clearly  its  priority  in  age.  It  composes  the  plateau  of  Virginia  City  and  Gold  Hill,  and 
derives  a  practical  interest  from  the  fact  that  the  Comstock  vein  is  enclosed  between 
propvlite  and  S3?enite,  though  in  some  parts  of  it  both  walls  consist  of  the  former  rock. 
Andesite  is  insignificant  in  bulk  in  that  region.  It  composes  a  few  small  hillocks  on 
the  propylitic  plateau,  and  in  some  cuts  and  tunnels  andesitic  dykes  may  be  seen,  which 
appear-to  have  been  the  feeding  channels  of  the  surface  accumulations.  Trachyte,  on 
the  contrary,  is  among  the  prominent  rocks  of  Washoe.  It  forms  a  high  and  rugged 
crest,  encircling  the  plateau  of  Virginia  and  Gold  Hill  to  the  east,  and  extending  for 
miles  to  the  north,  while  to  the  south  it  reappears  across  the  Carson  River.  No  evi- 
dence is  afforded,  in  Washoe,  for  the  establishment  of  the  mutual  relations  of  andesite 
and  trachyte,  while  it  is  conspicuous  that  the  latter  was  of  later  origin  than  propylite. 
Besides  the  evidence  offered  by  intersection  and  superposition,  another  fact  may  be 
noticed  which  is  suggestive  for  the  lengtb  of  the  period  that  elapsed  between  the 
eruptions  of  both  rocks.  It  is  this,  that  propylitic  sediments  occur,  at  least,  at  one 
thousand  feet  more  elevation  than  those  consisting  of  trachytic  matter,  which  fact 
appears  to  indicate  that  the  former  have  been  deposited  at  a  much  earlier  part  of  the 
period  marked  by  the  gradual  subsidence  of  the  inland  seas  of  the  Great  Basin  than 
has  been  the  case  with  trachytic  sediments. 

At  Silver  Mountain,  propylite,  of  the  augitic  variety,  fills  the  bottom  of  a  deep 
bisin  encircled  to  the  west  by  gra  litic  walls  several  thousuil  fe3t  in  hsight.  Its 
massive  accumulations  are  intersected  by  andesitic  and  trachytic  rocks,  which  latter 
appear  to  compose  the  summit  of  the  high  peak  of  Silver  Mountain  itself,  while  rhyo- 
lite  occurs  in  such  a  position  as  to  make  it  probable  that  it  has  arrived  at  the  surface 
last  of  all  eruptive  rocks.  Basalt  appears  to  occur  only  to  a  limited  extent  at  Silver 


"    Kiclitliofcn,  loc.  cit. 

(69) 


32  R1CIITHOFEN NATURAL    SYSTEM 

Mountain.  But  this  rock  is  largely  distributed  in  the  regions  adjoining  the  Sierra 
Nevada  to  the  east,  and  bears  evidence  of  its  recent  origin.  It  is  the  only  volcanic 
rock  which  covers  in  places  the  sand  of  the  deserts,  and  the  belts  distinguished  by  its 
eruptions  are  still  marked  by  the  occurrence  of  hot  springs  and  other  post-vol- 
canic phenomena.  Most  of  .these  are  contiguous  to  those  places  where  basalt  and 
granite  are  in  close  contact,  as  is  very  conspicuously  the  case  at  Steamboat  Springs, 
near  Washoe,  and  in  the  Coso  Mountains.  No  phenomena  of  similar  nature  appear 
to  be  connected  at  the  present  time  with  any  other  volcanic  rock  east  of  the  Sierra 
Nevada.  There  are  not  a  few  instances  where  basalt  may  be  seen  covering  propylite 
or  andesite  ;  but  I  met  with  only  one  case  where  it  comes  in  contact  with  rhyolite, 
close  enough  to  establish  their  mutual  relations.  This  is  in  Esmeralda,  on  the  eastern 
slope  of  the  Sierra  Nevada,  a  country  of  unusual  interest  for  the  study  of  volcanic 
rocks  in  general.  Propylite  encloses  the  silver-bearing  veins  of  that  place.  It  is 
overlain  by  trachyte  and  rhyolite,  both  of  which  occur  in  very  great  variety.  To 
the  east  of  the  place,  basalt  has  not  only  flowed  over  rhyolite,  but  contains  numerous 
fragments  of  it  enclosed,  which  fact  confirms  also  for  this  country  the  more  recent 
origin  of  basalt. 

Many  other  examples  might  be  added  to  this  short  list,  partly  of  positive  obser- 
vations made  in  the  countries  already  mentioned,  and  partly  of  facts  described  in 
treatises  on  the  geology  of  other  countries,  such  as  Armenia,  the  Caucasus,  Central 
France,  the  Eifel,  Bolivia,  Mexico.  As  these  descriptions,  however,  have  not  directly 
in  view  the  illustration  of  our  subject,  great  care  should  be  used  in  drawing  from 
them  conclusions  in  regard  to  it.  Let  it  suffice  to  remark,  that  every  observation  on 
record  which  bears  on  our  subject,  appears  to  confirm  the  proposed  law,  while  none 
can  be  found  giving  evidence  against  it.  This  may  justify  the  assumption  that  the 
periodical  succession  of  volcanic  rocks,  in  the  order  above  mentioned,  is  a  general  law, 
true  for  all  parts  of  our  planet. 

Laws  regarding  the  Mutual  Relations  of  Massive  Eruptions  and  Volcanic  Activity. 

It  has  been  mentioned  in  this  chapter,  that  the  law  of  the  periodical  succession  of 
volcanic  rocks  regards  those  outpourings  of  large  volumes  of  matter  not  resulting 
from  volcanic  activity  proper,  and  which  we  called  massive  eruptions.  For  conven- 
ience' sake,  we  make  use  of  the  following  expressions :  propylitic  epoch;  andesitic  epoch  ; 
trachytic  epoch ;  rhyolitic  epoch ;  basaltic  epoch  —  designating  thereby  those  epochs 
in  which  the  massive  eruptions  of  rocks  belonging  to  each  of  these  orders,  have  taken 
place  in  every  different  country.  If  we  now  direct  our  attention  to  the  other  mode 
of  manifestation  of  subterranean  energy,  the  volcanic  eruptions,  a  cursory  review  of 
active  volcanoes  in  regard  to  the  nature  of  the  rocks  which  they  eject,  shows  that  the 
same  law  is  no  longer  true  for  them,  as  their  lavas  belong  to  several  different  orders 
of  volcanic  rocks.  It  is,  however,  known  that  each  volcano  ejects  lava  or  scoria 
belonging  petrographically  only  to  one  distinct  order,  and  the  examination  of  the 
material  accumulated  by  former  activity  will  show  that  with  most  volcanoes  the 
nature  of  the  rocks  ejected  has  never  materially  varied,  while  with  some  of  the 

(70) 


OF    VOLCANIC    ROCKS.  33 

grander  vents  it  has  undergone  a  periodical  change.  Volcanoes,  whether  active  or 
extinct,  may  be  classified  from  this  point  of  view.  We  shall  distinguish:  andesitic, 
trachytic,  rhyolitic  and  basaltic  volcanoes,  according  to  the  nature  of  the  mineral  matter 
which  each  volcano  has  ejected  in  the  first  epoch  of  its  activity,  regardless  of  any  later 
changes.  Two  noteworthy  relations  may  be  traced  between  these  different  orders 
of  volcanoes  and  the  massive  eruptions  of  the  synonymous  orders  of  volcanic  rocks. 
The  first  of  them  is  the  alliance  of  both  in  regard  to  geographical  distribution,  the 
volcanoes  of  each  order  being  limited,  in  this  respect,  to  the  immediate  neighborhood 
of  massive  accumulations  of  rocks  similar  in  nature  to  their  first  lavas.  From  this 
may  partly  be  inferred  the  second  relation,  that  the  massive  eruptions  of  each  order 
have  been  succeeded  by  volcanic  activity,  which  occasioned  the  ejection  of  lava  corre- 
sponding in  nature  to  their  own  rocks,  and  continued  for  long  after-time,  in  many  instan- 
ces to  the  present  day.  This  dependence  of  volcanoes  upon  massive  eruptions  explains 
why  the  number  of  active  volcanoes  is  so  small  when  compared  with  those  which 
are  extinct,  and  why  the  present  activity  even  of  those  which  are  still  in  operation,  ap- 
pears to  be  only  a  faint  remnant  of  that  which  the  same  vents  exhibited  in  former  time. 
It  will  further  explain  why  no  vestige  can  be  found  of  a  rhyolitic  volcano  having  been 
active  before  the  rhyolitic  epoch,  or  of  a  basaltic  volcano  having  originated  before  the 
basaltic  epoch,  while  geological  observation  goes  to  show  that  during,  and  immediately 
after  those  epochs,  the  volcanoes  of  either  order  have  been  most  intense,  numerous 
and  extensive,  and  their  activity  has,  from  that  epoch  of  culmination,  gradually  relaxed, 
in  most  cases  to  perfect  extinction. 

An  instructive  instance  of  one  of  those  grander  volcanoes  which  have  undergone 
a  periodical  change  in  regard  to  the  nature  of  the  matter  ejected  from  them,  is  afforded 
by  the  extinct  volcano  Lassen's  Peak,  in  Northern  California,  which  Professor  J.  D. 
Whitney  and  I  visited  in  1866.  We  found  it  to  have  been  originally  an  andesitic 
volcano,  and  it  has  to  be  ranked  as  such  in  our  proposed  classification.  The  enormous 
bulk  of  the  ancient  volcano  is  totally  built  up  of  stratified  layers  of  andesitic  tufa 
and  rapilli,  which,  in  the  steep  gorge  issuing  from  its  lower  crater,  are  exposed  in  a 
thickness  of  nearly  four  thousand  feet,  notwithstanding  the  total  destruction  which 
the  upper  part  of  the  former  cone  has  undergone,  and  the  fact  of  its  lower  parts 
extending  down  far  beneath  the  present  surface,  and  being  therefore  concealed  to  view. 
Besides  these  stupendous  accumulations  of  loose  matter,  currents  of  andesitic  lava 
appear  to  have  been  emitted  from  the  crater,  extending  at  ,least  twenty  miles  from 
the  place  of  ejection.  At  a  later  epoch,  the  activity  of  the  same  volcano  has  been 
distinguished  by  the  emission  of  trachytic  lava  from  the  northeastern  part  of  the 
wall  of  the  crater  ;  its  currents  have  expanded  to  elongated  and  sloping  tables, 
bounded  by  abrupt  descents.  A  third  epoch  is  marked  by  the  outbreak  of  rhyolite 
at  the  same  place  whence  the  trachytic  rocks  had  issued.  Rhyolite  composes  the 
present  summit  of  Lassen's  Peak,  on  which  it  is  accumulated  in  a  thickness  of  more 
than  fifteen  hundred  feet,  also  some  other  summits  of  less  altitude,  and  at  least  one 
prominent  current  of  lava  of  great  volume.12  The  noteworthy  fact  illustrated  by  these 
observations  on  Lassen's  Peak,  and  corroborated  in  numerous  other  instances,  is  this : 

12  Mr.  Clarence  King  has  observed  the  occurrence  of  basalt  of  apparently  very  recent  origin  immediately  north  of 


34  RICHTHOFEN —  NATURAL    SYSTEM 

that  the  same  law  of  periodical  succession  which  has  been  established  in  regard  to 
massive  eruptions,  is  true  for  volcanic  action,  particularly  when  this  happened  to 
assume  such  unusual  intensity  and  dimensions,  and  has  been  of  as  long  duration  as 
was  the  case  at  that  volcano.18 

Summing  up  these  considerations  on  the  correlation  of  the  different  orders  of 
volcanic  rocks  in  respect  to  the  age  of  their  emission  through  volcanic  vents,  we  arrive 
at  the  following  conclusion  :  The  commencement  of  the  activity  of  the  volcanoes  of 
each  separate  order  has  been  nearly  coincident  with,  though  in  every  instance  success- 
ive to,  the  main  phase  of  the  corresponding  massive  eruptions.  Thence  it  has,  by  each 
separate  vent,  either  continued  emitting  similar  material  to  that  first  ejected,  until  its 
extinction,  or  it  continued  in  the  same  way  to  the  present  day,  or  it  has  been  subjected 
to  a  periodical  change  in  regard  to  the  nature  of  its  lavas,  and  this  change  is  analogous 
to  that  exhibited  by  the  succession  of  massive  eruptions.  In  this  case,  as  in  the  for- 
mer, the  volcano  has  either  become  extinct  when  in  a  certain  phase,  or  it  is  still  active. 
We  are  thus  furnished  with  a  natural  cause  of  the  fact,  that  most  active  volcanoes  are 
emitting  basaltic,  a  smaller  number  of  them  rhyolitic  or  trachytic  rocks,  while  an- 
desitic  lava  is  peculiar  only  to  a  few  of  them,  especially  to  some  of  the  prominent  vol- 
canoes of  South  America  (Chimborazo,  Cotopaxi,  Antisana,  Tungurahua,  also  Popoca- 
tepetl, Colima,  and  TeneritFe)  which  appear  never  to  have  changed  in  mineral  char- 
acter. It  will,  too,  be  self-evident,  why  generally  no  material  change  in  the  nature 
of  their  lava  should  have  been  observed  in  regard  to  those  volcanoes  which  have  orig- 
inally emitted  basalt  and  constitute  our  order  of  basaltic  volcanoes. 

A  few  more  instances  may  here  be  mentioned  in  support  of  our  propositions. 
The  interest  attaching  to  volcanoes  has  furnished  us  with  a  much  greater  number 
of  facts  in  regard  to  volcanic  rocks  when  occurring  as  lavas,  than  we  possess  in  regard 
to  the  grander  and  more  frequent  instances  when  similar  rocks  have  been  produced 
by  the  comparatively  neglected  action  of  massive  eruptions.  Among  those  observa- 
tions, none  will  be  better  evidence  than  such  as  prove  the  abrupt  succession,  by  ejec- 
tion from  the  same  volcano,  of  two  rocks  so  dissimilar  in  composition  as  rhyolite  and 
basalt.  On  the  other  hand,  the  nature  of  volcanic  action  will  explain  why  we  should 
meet  among  lavas,  more  frequentl}'  than  among  massive  eruptions,  with  the  fact  of 
two  successive  epochs  blending  into  each  other  by  the  alternation  of  the  two  kinds  of 
rock  peculiar  to  them  separately,  and  it  cannot  be  surprising  if  instances  are  occasion- 
ally observed  exhibiting,  at  least  partly,  a  reversed  order  of  succession. 


Lissen's  Peak.  It  is  probable  that  it  indicates  the  existence  of  a  fourth  epich  in  the  activity  of  that  volcano.  The  ejection 
of  basalt  has  been  so  frequently  connected  with  the  opening  of  vents  in  the  neighborhood  of,  but  not  coinciding  with,  channels 
through  which  its  predecessors  had  ascended,  that  its  local  Reparation  cannot  l>e  an  argument  against  its  belonging,  in  our 
ca-e,  to  the  system  of  Lassen's  Peak. 

13  The  fact  that  trachytic  lavas  are  frequently  followed  by  such  of  basaltic  character  has  been  known  since  long 
time,  and  was  till  now  the  only  law  of  succession  observed.  Mr.  Scrope  has  suggested  the  hypothesis  indorsed  by  Mr.  Dar- 
win, Sir  Charles  Lyell,  and  other  distinguished  geologists,  that  in  the  subterranean  reservoirs  of  volcanic  matter,  the  heavier 
particles  will  occupy  the  lower  part,  and  the  lighter  ones  be  nearer  the  earth's  crust.  It  will  easily  be  seen  how  totally 
inadmissible  this  theory  is  in  the  case  of  Lassen's  Peak.  It  is  not  less  so  in  those  cases  where  rhyolite  was  succeeded  by 
basalt,  since  the  process  of  liquation  can  certainly  not  be  supposed  to  have  produced  an  abrupt  passage  under  ground  from 
one  mass  to  the  other,  nnd  it  would  be  much  more  natural  to  suppose  a  gradual  transition  to  take  place,  there  as  well  as  in 
the  succession  of  the  rocks  emitted  to  the  surface,  if  liquation  had  really  taken  place. 
(72) 


OF    VOLCANIC    ROCKS.  35 

Au  instance,  which  is  instructive  on  account  of  its  simplicity,  is  furnished  by 
the  island  of  St.  Paul,  in  the  Indian  Ocean.  F.  von  Hochstetter  found  its  foundation 
to  consist  of  rhyolitic  rocks.  These  are  intersected  by  basaltic  dykes.  Rhyolite  over- 
lays the  first  basaltic  formation,  and  is  itself  superposed,  first  by  dolerite  and  then 
again  by  basalt.  These  two  rocks  of  the  basaltic  order  constitute  the  main  body  of 
the  island,  and  encircle  its  crater.  Similar  relations,  though  on  a  much  grander  scale, 
have  been  observed  by  the  same  eminent  geologist  on  New  Zealand.  More  frequently 
than  this  order  of  succession  between  rhyolitic  and  basaltic  lava,  has  been  observed 
the  sequence  of  basalt  to  trachyte,  with  the  omission  of  rhyolite,  or  immediately  to 
andesite.  when  both  those  rocks  are  absent.  Vesuvius  is  built  up  of  rocks  of  the 
basaltic  order,  and  still  emits  lava  corresponding  in  mineral  character  to  its  predeces- 
sors, while  the  rocks  of  its  surroundings  (Campi  Phlegrcei),  on  the  prior  origin  of 
which  geologists  agree,  are  trachytic.  The  industrious  explorer  of  Mount  Etna,  Sarto- 
rius  von  Waltershausen,  has  described  its  foundation  as  being  composed  of  white  and 
reddish  colored  trachytic  rocks,  which  contain  hornblende  as  a  characteristic  ingred- 
ient, while  among  those  rocks  which  build  up  the  summit,  as  also  in  all  modern  lava  of 
the  volcano,  no  hornblende  but,  in  its  place,  augite  is  visible.  This  mineral  and  labra- 
dor  compose  the  recent  lava,  which  belongs  to  the  basaltic  order.  The  much  more 
extensive  recurrence  of  a  similar  order  of  succession  in  the  Eifel  and  in  Auvergne, 
is  too  well  known  from  the  accurate  descriptions  of  the  geology  of  those  regions,  to 
require  to  be  here  more  fully  mentioned.  It  contributes  especially  to  confirm  our 
proposition,  that  the  volcanoes  of  the  different  orders,  as  regards  their  origin,  have 
been  nearly  contemporaneous  with  the  correlated  massive  eruptions.  The  classical 
descriptions  of  the  Eifel,  by  Mr.  von  Dechen,  give  conclusive  evidence  thereof. 

Among  those  volcanoes  the  lava  of  which  has  never  undergone  a  material 
change  and  is,  at  the  same  time,  similar  in  nature  over  the  area  of  larger  volcanic 
districts,  may  be  mentioned,  besides  numerous  basaltic  volcanoes,  those  of  the  trachytic 
order  in  Central,  and  those  of  the  andesitic  order  in  South  America,  as  far  as  may  be 
seen  from  the  descriptions  given  of  them. 

We  might  greatly  enlarge  this  enumeration  of  observations  confirming  our 
propositions  :  but,  as  by  most  authors  only  a  "  trachytic"  or  a  "  basaltic"  character 
of  lava  have  been  mentioned  in  a  general  way,  they  would  only  furnish  evidence  in 
favor  of  the  general  tenor  of  the  law,  but  would  fail  to  give  it  in  regard  to  any  of  its 
details. 

RELATION  OF  VOLCANIC   ROCKS  TO  ANCIENT  ERUPTIVE  ROCKS. 

All  rocks  which,  bearing  evidence  of  an  intrusive  or  eruptive  origin,  preceded 
in  age  the  Tertiary  period,  may,  by  principles  similar  to  those  which  we  applied  in 
tracing  the  natural  system  of  volcanic  rocks,  be  divided  into  two  great  classes,  for 
which  we  may  use  the  terms  "granitic  rocks"  and  "  porphyritic  rocks,"  derived  from 
the  mode  of  texture  predominating  in  either  class.  Granitic  rocks  are,  besides, 
geologically  associated  with  granite,  which  is  their  principal  type,  while  quartzose 
porphyry  occupies  a  similar  position  among  porphyritic  rocks.  The  annexed  table 

(73) 


36 


RICHTIIOFEN  —  NATURAL    SYSTEM 


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(74) 


OF   VOLCANIC   ROCKS.  37 

will  show  the  mutual  relation  of  these  two  classes  and  their  subdivisions,  and  of 
either  of  them  to  volcanic  rocks.14 

It  appears  that  this  general  classification  is  based  upon  as  natural  principles  as 
are  within  reach  of  our  still  limited  knowledge  of  eruptive  rocks,  and  therefore  may 
at  least  approach  the  natural  system.  The  following  are  the  systematical  principles 
chiefly  involved  : 

1st.  Chemical  Composition, — Each  class  contains  all  possible  compounds  inter- 
mediate between  those  which  Bunsen  styled  the  normal  trachytic  and  normal  pyroxe- 
nic  types,  and  may,  therefore,  be  represented  by  a  numerical  series  of  infinite  grada- 
tions within  two  certain  limits,  and  progressing  according  to  a  definite  arithmetical 
law.  If  this  law  applied  to  the  composition  of  rocks  with  mathematical  precision,  it 
would  be  sufficient  to  know  the  relative  quantity  in  which  any  one  single  ingredient 
enters  into  the  same,  in  order  to  find  by  calculation  the  relative  proportion-in  which 
every  other  ingredient  should  be  present.  It  is,  however,  well  known,  that  analysis 
shows  ordinarily  a  slight  deviation  from  the  composition  as  required  by  theory  ;  and, 
considering  the  various  influences  to  which  the  rocky  masses  must  have  been  exposed 
before  and  after  their  consolidation,  we  should  naturally  presuppose  that  such  devia- 
tions would  be  the  rule,  and  may  indeed  be  astonished  to  see  how  slight  they  generally 
are.  Silica  has  been  found  to  be  not  only  the  most  convenient,  but  also,  on  account 
of  its  predominance  over  other  ingredients,  the  safest  element  by  which  to  determine 
the  place  any  rock  occupies  in  the  series.  In  the  classification  as  proposed  in  the  pre- 
ceding page,  each  class  of  eruptive  rocks  commences  with  those  varieties  of  granite, 
quartzose  porphyry  and  rhyolite,  which  contain  the  highest  amount  of  silica  as  found 
by  analysis,  and  descends  to  the  most  basic  varieties  of  diabase,  augitic  porphyry  and 
basalt.  In  a  chemical  respect,  therefore,  the  three  classes  are  identical.15 

14  This  table  is  only  designed  to  show  the  mutual  relations  of  the  subdivisions  of  the  three  great  classes  of  erup- 
tive rocks  in  their  most  general  outlines.  I  have  purposely  avoided  to  detail  them  any  more,  since  it  appears  that  great 
progress  in  regard  to  their  knowledge  will  be  made  in  the  next  years,  and  considerable  changes  in  the  details  of  classi- 
fication may  have  to  be  looked  for.  The  composition  of  eruptive  rocks  is  just  at  the  present  time  being  made  an  object  of 
more  careful  study  than  it  ever  was  before.  Contributions  of  high  value  in  regard  to  the  chemical  and  mineral  com- 
position of  volcanic  rocks  are  being  furnished  by  the  members  of  the  Geological  Institute  of  Vienna ;  while  H.  Abieh 
continues  with  untiring  energy  his  fruitful  researches  on  the  same  rocks  of  Armenia,  the  Caucasus,  and  the  borders  of  the 
Caspian  Sea.  The  examination  of.  the  granitic  rocks  by  G.  voru  Rath,  A.  Strong,  Th.  Scheerer  and  many  others;  the 
more  general  labors  of  G.  Rose,  Robert  Bunsen,  A.  Delesse,  G.  BishofF,  P.  Zirkel ;  the  microscopic  investigations  which 
have  been  commenced  with  so  much  success  by  A.  Sorby,  and  are  being  extended  by  his  numerous  followers — all  these 
labors  pursued  zealously  by  those  named  and  a  great  many  other  workers  in  the  same  branch  of  geological  science,  whom  it 
would  be  too  lengthy  to  mention,  promise  a  rapid  advance  of  our  knowledge  of  the  properties  of  eruptive  rocks.  In 
respect  to  their  mineral  composition,  the  discovery  of  some  new  species  of  feldspar  (such  as  the  plagioclase  of  Rath  and  the 
microtine  of  G.  Tschermak)  which  appear  to  be  of  wide  distribution  among  the  components  of  eruptive  rocks,  promises 
alone  to  enlarge  quite  considerably  the  basis  of  classification,  as  it  appears  to  give  a  clue  to  the  causes  of  the  differences  in 
outward  appearance  (frequently  referred  to  in  this  paper)  of  rocks  which  are  alike  in  regard  to  their  chemical  composition, 
and  among  the  mineral  ingredients  of  which  no  difference  could  be  recognized  heretofore. 

5  Objections  have  been  raised  against  the  validity  of  the  law  of  Bunsen,  partly  on  the  ground  of  the  frequent 
discrepancy  of  the  figures  obtained  by  chemical  analysis  from  those  which  would  be  required  by  the  theory,  and  partly  on 
account  of  the  highest  and  lowest  amounts  of  silica  occurring  in  eruptive  rocks  having  as  yet  by  no  means  been  fully 
ascertained.  We  referred  above  to  the  first  objection.  As  regards  the  second,  it  is  evidently  very  trifling  in  importance. 
The  limits  of  the  series  may,  and  probably  will,  bo  somewhat  extended,  and  the  figures  representing  the  compounds  which 
form  those  limits  may  have  to  be  slightly  changed,  yet  the  series  itself  will  remain  essentially  the  same.  The  law  of  Bun- 
sen  will  have  to  be  revised  and  corrected  when  increased  experience  shall  have  established  a  broader  basis  for  it ;  but  no 
change  of  its  principles  may  ever  be  exacted,  as  an  overwhelming  amount  of  evidence  has  accumulated  in  support  of  its 
essential  tenor.  j  (75) 


38  RICHTHOFEN  —  NATURAL    SYSTEM 

2d.  Mineral  composition,  in  respect  to  which  each  class  represents  a  series  of 
gradations  essentially  dependent  on  the  chemical  composition,  and  therefore  chiefly 
coincident  with  the  chemical  series,  but  differing  from  it  inasmuch  as  it  is  more  articu- 
late. Certain  types,  corresponding  to  certain  steps  in  the  chemical  scale,  and  consist- 
ing in  distinct  aggregations  of  a  few  minerals,  are  the  centers  for  which  the  petro- 
graphical  names  have  been  applied  in  the  first  place  ;  around  them  other  members  are 
grouped  which  connect  every  two  types  by  gradual  passages,  and  are  ordinarily  com- 
posed of  an  aggregation  of  all  the  minerals  peculiar  to  either  of  them.  Expansion  of 
the  series,  as  it  were,  in  a  lateral  sense,  occasioned  by  the  accession  of  minerals  inferior 
in  importance,  are  not  unfrequent,  but  do  not  affect  in  a  great  measure  the  definite 
character  of  mineralogical  gradation,  as  they  are  of  local  occurrence,  and  probably  less 
dependent  upon  any  material  differences  in  chemical  composition,  than  upon  certain 
influences  which  acted  upon  the  mass  of  the  rock,  either  before  its  ejection,  and 
then  by  the  admixture  of  matter  differing  from  it  in  nature,  or  after  its  solidification, 
and  then  by  chemical  metamorphism. 

3d.  Specific  gravity,  which  increases  in  the  reverse  proportion  of  silica.  In  this 
respect,  too,  each  class  represents  a  series  of  infinite  gradations  from  the  lowest  to  the 
highest  value. 

Taking  the  three  foregoing  principles  exclusively  as  the  basis  for  classification, 
all  eruptive  rocks  would  have  to  be  united  into  one  class.  This  union,  made  regard- 
less of  any  other  relations,  may  be  traced  as  the  leading  feature  of  nearly  all  systemat- 
ical arrangements  proposed.  There  are,  however,  other  points  of  view  which  must  be 
considered  if  a  more  perfect  classification  is  aspired.  They  lead  to  the  establishment 
of  further  separations,  by  principles  similar  to  those  which  we  had  to  apply  above  in 
defining  the  different  orders  of  the  compounds  of  hornblende  and  oligoclase  among 
volcanic  rocks.  These  points  of  view  are  the  following  : 

4th.  Mode  of  Texture. — Eruptive  rocks  exhibit,  in  regard  to  their  modes  of  text- 
ure, very  peculiar  differences,  which  are  little  capable  of  explanation  with  our  present 
state  of  knowledge.  They  are  especially  conspicuous  with  the  most  silicious  rocks. 
Free  silica,  in  granite,  has  probably  been  solidified  cotemporaneously  with  the  other 
component  minerals  ;  but  its  solidification  appears  often  to  have  been  completed  last 
of  all,  as  the  quartz  does  envelop  the  aggregation  of  the  other  crystals.  In  quartzose 
porphyry  and  rhyolite,  on  the  other  hand,  free  silica,  at  least  the  greater  part  of  it, 
has  been  solidified  first,  which  is  obvious  from  the  fact  that  its  perfect  crystals  are 
imbedded  in  an  ordinarily  microcrystalline  aggregation  of  the  other  ingredients,  which 
however  contains,  in  most  cases,  some  surplus  of  free  silica  that  had  not  entered  into 
the  composition  of  the  crystals.  This  fundamental  difference  from  granite  points  to 
some  difference  in  the  mode  of  origin.  Quartzose  porphyry  and  rhyolite  differ  in  regard 
to  the  texture  of  their  paste,  which  has  a  compact  aspect  in  the  first,  while  it  is  more  or 
less  vesicular  throughout  the  mass  in  most  varieties  of  the  latter.  This  difference,  like 
the  former,  appears  to  indicate,  that  the  molecular  condition  of  the  liquid  mass,  at  the 
time  when  it  was  ejected,  was  different  in  either  case.  The  variety  of  texture  diminishes 
with  the  decrease  of  silica,  and  the  more  basic  rocks  of  the  three  classes  bear  a  much 

(76) 


OF   VOLCANIC    ROCKS.  39 

closer  resemblance  to  each  other  than  the  silicious  rocks.  The  cause  must  be,  either 
that  the  conditions  of  the  mass  before  cooling  were  less  varied  with  basic  than  with 
silicious  rocks,  or  that  the  differences  were  alike,  but  manifested  themselves  less  con- 
spicuously in  the  character  of  the  solidified  rocks  ;  the  latter  cause  is  the  most  probable 
of  the  two,  since  even  our  cotemporaneous  basaltic  and  andesitic  lavas  offer  but  a  small 
number  of  varieties  comparing  with  those  of  rhyolite  or  trachyte,  although  it  is  not 
likely  that  there  is  any  more  variety  among  the  circumstances  modifying  the  latter  than 
among  those  which  are  acting  upon  the  former.  It  is  chiefly  the  similarity  in  texture 
which  has  occasioned  the  general  application  of  such  names  as  "trap,"  "greenstone," 
"  aphanite,"  and  others,  for  basic  rocks  of  all  ages,  while  those  containing  a  large 
amount  of  silica  have  scarcely  ever  been  similarly  confounded.  We  may,  however,  note 
this  difference  among  the  former,  that  granitic  texture — that  is,  a  crystalline  aggrega- 
tion of  the  component  minerals — is  peculiar  to  those  basic  rocks  associated  with  granite, 
while  the  vesicular  or  trachytic  texture  is  only  proper  to  volcanic  rocks.  Occasionally, 
though  very  rarely,  rocks  of  granitic  texture  are  geologically  associated  with  porphy- 
ritic  rocks,  as  is  the  case  near  Predazzo,  in  southern  Tyrol. 

5th.  Geological  aye  ;  granitic  rocks  being  generally  the  most  ancient  in  origin, 
volcanic  rocks  the  most  recent,  while  those  of  porphyritic  structure  are  intermediate  in 
age  between  both. 

6th.  Other  geological  conditions  resulting  from  the  correlation  of  the  different 
foregoing  principles,  the  outlines  of  which  I  will  attempt  to  trace  in  the  following 
pages. 

Correlation  of  Age  and  Texture. 

It  is  scarcely  possible  to  treat  of  one  of  the  foregoing  principles  in  its  appli- 
cation to  eruptive  rocks  singly,  without  constantly  encountering  points  of  intimate 
connection  with  others,  so  thoroughly  are  they  intertwined  and  mutually  dependent. 
It  is  the  object  of  petrology  to  develop  these  correlations.  We  contemplate  them 
here  only  in  their  purely  geological  bearings,  in  order  to  try  to  establish  the  true  rela- 
tion of  volcanic  rocks  to  their  ancient  predecessors.  We  have,  while  occupied  with 
these  considerations,  constantly  to  keep  in  view  how  few  are  the  observations  upon 
the  strength  of  which  we  have  to  base  conclusions.  The  area  on  the  globe  whose 
special  geology  has  been  made  the  object  of  investigation,  though  extending  every 
year,  is  still  very  limited  ;  and  even  in  the  best  explored  countries,  little  attention 
has,  in  most  instances,  been  paid  to  the  distinction  of  eruptive  rocks.  Observations  in 
regard  to  them  are  abundant  in  some  parts  of  Europe,  fortunately  in  such  countries 
as  are  especially  capable  of  giving  a  clue  to  their  general  knowledge.  Distinct 
conclusions  may  be  arrived  at  in  regard  to  portions  of  that  continent,  but  little  scope 
is  afforded  for  giving  them  latitude  by  comparison  with  the  i-elations  presented  in  other 
countries.  We  have,  therefore,  to  keep  well  separate  in  general  petrology,  those  posi- 
tive conclusions  which  are  founded  upon  sufficient  observations,  and  are  applicable  to 
the  relations  in  those  countries  where  the  latter  were  made,  and  the  realm  of  theories, 
which  are  arrived  at,  partly  by  the  generalization  of  those  conclusions,  and  partly  by 
making  deductions  from  hypothetical  suppositions;  because,  the  premises  being  founded 

(77) 


40  RICIITIIOFEN NATURAL   SYSTEM 

on  local  occurrences,  and  their  general  validity  not  being  proved,  the  theories  must 
necessarily  have  a  great  deal  of  uncertainty,  which  will  only  gradually  be  dispelled  by 
the  advancing  knowledge  of  the  geology  of  the  globe. 

The  correlation  of  age  and  texture,  as  resulting  from  observations  made  in 
Europe,  will  occupy  us  first.  Granitic  rocks  are  widely  distributed  on  that  conti- 
nent. Their  great  eruptive  masses  are  of  Azoic  and  Paheozoic  age.  The  rocks 
have  almost  throughout  granitic  texture,  though  the  distinguishing  features  of 
porphyritic  rocks  are  proper  to  some  subordinate  varieties  of  diorite  and  diabase. 
The  eruptive  activity  exhibited  in  the  granitic  era  gradually  relaxed.  It  appears 
to  have  been  insignificant,  though  it  was  by  no  means  extinct,  in  the  latter  part 
of  the  Devonian  and  the  first  part  of  the  Carboniferous  periods.  But  it  recom- 
menced about  the  time  of  the  deposition  of  the  coal  measures,  thence  increased  in  in- 
tensity, and  was  most  violent  in  the  Permian  age,  though  it  was,  during  all  that 
time,  much  more  limited  in  extent  than  it  had  been  during  the  granitic  era.  The  mid- 
dle portions  of  Germany  were  then  its  principal  theater,  until  it  changed,  in  the  com- 
mencement of  the  Triassic  age,  to  the  southern  slope  of  the  Alps  and  Carpathians. 
The  rocks  produced  during  this  era  possess,  almost  exclusively,  porphyritic  texture. 
The  Jurassic,  Cretaceous,  and  the  first  part  of  the  Tertiary  ages,  were  distinguished 
by  an  almost  complete  repose  of  eruptive  action,  in  Europe.  It  was  only  after  the 
commencement  of  the  Tertiary  period  when  that  violent  resumption  of  eruptive  ac- 
tivity took  place  to  which  we  have  repeatedly  called  attention  in  the  foregoing  pages, 
and  which  thereafter  continued,  gradually  relaxing,  down  to  our  present  era,  the  mani- 
festations of  volcanic  action  in  which  are'its  last  faint  remnant.  Vesicular  inflation, 
which  is  the  characteristic  feature  of  trachytic  texture,  is  peculiar  to  the  rocks  of  this 
class  ;  all  of  them  possess  it  more  or  less,  though  there  are  varieties  resembling  por- 
phyritic rocks  closely  in  aspect.  A  combination  of  the  trachytic  and  porphyritic  modes 
of  texture  is  of  more  common  occurrence  than  either  of  them  singly,  and  is  indeed 
the  distinguishing  feature  of  volcanic  rocks. 

These  relations  of  age  and  texture  have  been  conclusively  proved  to  exist  in 
Europe,  and  appeared  to  justify  the  conclusion,  that  the  three  classes  of  eruptive  rocks 
are  geologically  separated,  and  represent  three  successive  and  distinct  phases  of  the 
manifestation  of  subterranean  agencies.  Considering  in  their  generality  the  facts  ob- 
served in  other  countries,  they  appeared  to  confirm  these  views,  since  granitic  rocks 
are  known  to  be  generally  very  ancient ;  volcanic  rocks  to  have  been  generated  during 
the  Tertiary  and  Post-tertiary  periods  ;  while,  in  regard  to  porphyritic  rocks,  observations 
have  been  scarce,  and  descriptions  lack  distinctness,  yet  no  relations  have  been  recorded 
in  reference  to  them  which  would  be  contradictory  to  those  observed  in  Europe.  On  the 
western  coast  of  North  America,  however,  eruptive  rocks  exhibit  some  peculiar  rela- 
tions— differing  to  some  extent  from  those  which  they  olfer  in  Europe.  Positive  facts 
are  known  sparsely  outside  of  California,  but  conditions  similar  to  those  observed  in 
that  country  appear  to  be  repeated  through  large  portions  of  the  range  of  the  Andes. 
Prof.  J.  D.  Whitney's  admirable  researches  on  the  age  of  the  metamorphic  rocks  of 
the  Sierra  Nevada  have  clearly  demonstrated  that  the  granitic  rocks,  which  partake  to 

(78) 


OF   VOLCANIC    ROCKS.  41 

great  extent  in  the  structure  of  that  mountain  range,  cannot  possibly  have  been 
ejected  prior  to  the  Jurassic  epoch.  The  texture  of  these  rocks  is,  notwithstanding 
this  comparatively  recent  origin,  that  of  all  true  granite,  and  the  prominent  varieties 
cannot  be  distinguished  from  some  European  kinds  of  granite,  as  for  instance  those  of 
the  Adamello  arid  the  Cima  d'Asta  in  the  Southern  Alps,  which  are  among  the  most 
recent  in  age  on  that  continent.  Volcanic  rocks  are  widely  distributed  in  the  Sierra 
Nevada,  and  are  of  the  same  or  similar  age  as  in  Europe.  Quartzose  porphyry  occurs 
to  some  extent  in  Washoe,  under  circumstances  which  make  the  exact  determination 
of  its  age  difficult,  but  render  it  certain  that  it  is  intermediate  in  this  respect  between 
granitic  and  volcanic  rocks.  These  relations  would  appear  to  be  an  exact  counterpart 
of  those  observed  in  Europe,  with  the  one  prominent  difference,  that  the  commencement 
of  the  eruptive  action  was  much  later  in  America.  Very  recently,  however,  additional 
observations  have  been  made,  which  give  a  somewhat  different  aspect  to  these  relations. 
Mr.  Clarence  King  observed  granite,  covered  by  Pakeozoic  rocks  and  antecedent  to 
them  in  age,  near  the  Colorado  River  ;  while  Prof.  Whitney  and  myself  discovered  true 
quartzose  porphyry  in  the  county  of  Plumas,  in  northern  California,  associated  with 
rocks  proved  by  the  former  to  be  of  Triassic  and  Liassic  age,  in  such  way  as  to  leave 
little  doubt  about  its  cotemporaneous  origin.  Farther  east,  in  the  Great  Basin, 
Palaeozoic  granite  is  of  no  rare  occurrence,  and  it  is  among  the  prominent  features  in 
the  geology  of  the  Rocky  Mountains  ;  while  the  discovery  of  porphyritic  rocks  may 
have  to  await  further  examination,  they  having  been  in  most  countries  the  last  erup- 
tive rocks  to  be  detected. 

However  these  facts  may  affect  the  theoretical  conclusions  in  regard  to  the 
origin  and  mutual  relations  of  granitic  and  porphyritic  rocks,  which  had  been  made  on 
the  strength  of  former  observations,  they  appear  to  confirm  the  separation,  from  a  geolo- 
gical point  of  view,  of  both  classes  of  rocks.  There  has  been  in  the  Sierra  Nevada  and 
adjacent  countries,  it  appears,  an  ancient  granitic  era  corresponding  to  that  of  Europe, 
followed  by  a  porphyritic  era  which  was  nearly  or  quite  coincident  with  the  European. 
But,  while  the  manifestations  of  subterranean  agencies  almost  ceased  in  Europe  during 
the  following  ages,  they  recommenced  with  great  intensity  on  the  western  coast  of 
North  America,  and  gave  rise  to  a  second  granitic,  followed  by  a  second  porphyritic  era. 
The  volcanic  era  commenced  in  both  countries  in  the  Tertiary  epoch,  but  it  appears  to 
have  been  in  an  advanced  stage  in  Europe  while  it  was  still  in  its  birth-throes  in 
America. 

Correlation  of  Age  and  Composition. 

This  point  of  view  is  not  inferior  in  interest  to  the  foregoing.  The  most 
noteworthy  fact  is  this,  that  quartzose,  and  in  general  highly  silicious  rocks  prevail 
among  those  of  ancient  origin,  basic  compounds  among  those  of  later  ages.  Granite 
and  syenite  are  overwhelmingly  predominant  among  ancient  eruptive  rocks.  Diorite 
and  diabase  are  generally  associated  with  them,  but  remain  always  quite  subordinate 
in  bulk.  The  relative  proportion  is  different  with  porphyritic  rocks.  So  little 
attention  has  been  paid  to  these,  outside  of  Europe,  that  general  conclusions  in 

(79) 


42  RICHTHOFEN NATURAL   SYSTEM 

regard  to  them  should  be  drawn  with  care.  In  the  middle  part  of  Germany, 
and  in  southern  Tyrol,  where  they  have  been  repeatedly  studied,  subjected  to 
chemical  analysis,  and  described  in  numberless  treatises,  quartzose  porphyry  is 
rather  predominant.  But  porphyrite,  melaphyr,  and  augitic  porphyry,  are,  in  the 
aggregate,  little  subordinate  in  bulk.  The  volcanic  offers  the  complete  reverse 
of  the  granitic  era,  respecting  the  proportionate  quantity  in  which  the  different  com- 
pounds have  come  to  the  surface.  Andesite  and  basalt  compose  as  large  a  proportion 
of  the  aggregate  bulk  of  volcanic  rocks,  as  granite  and  syenite  do  of  those  of  the 
granitic  era. 

Some  minor  differences  in  age  may  be  noticed  among  the  different  orders  com- 
posing the  three  classes  of  eruptive  rocks.  Extrusions  of  granite  and  syenite  appear 
to  have  been  almost  the  exclusive  feature  of  the  eruptive  activity  during  granitic  eras, 
and  to  have  been  succeeded  only  towards  their  close  by  the  emission  of  diorite  and 
diabase,  or  of  other  rocks  of  limited  occurrence,  such  as  gabbro  and  hypersthenite. 
Such  at  least  has  been  observed  to  be  the  case  in  several  countries,  in  regard  to  the 
chief  outbreaks  ;  but  if  we  enter  into  the  details  of  the  mode  of  succession  of  the 
rocks  belonging  to  the  different  orders,  we  perceive  that  it  has  not  been  so  definite 
as  with  volcanic  rocks — granite  and  syenite  bearing  evidence,  in  many  localities,  of  a 
more  recent  age  than  some  neighboring  masses  of  basic  rocks.  Yet,  in  keeping  only 
the  main  features  in  view,  we  may  easily  see,  that  the  general  order  of  succession  of 
granitic  rocks  has  been  conformable  to  a  gradual  decrease  in  silica.  Syenite  is  usually 
more  recent  in  origin  than  granite ;  and  even  among  the  different  varieties  of  the  lat- 
ter, true  granite,  containing  the  highest  ratio  of  silica,  has  generally  been  anterior  in 
age  to  G.  Rose's  granitite.  There  may  be  some  connection  between  these  relations 
as  they  are  exhibited  in  any  single  granitic  district,  and  the  fact  that  the  granitic  rocks 
of  the  Sierra  Nevada,  belonging  altogether  to  a  more  recent  era,  contain  no  true  granite — 
their  chief  bulk  consisting  of  rocks  intermediate  in  composition  between  granitite  and 
syenite.  The  porphyritic  era,  in  Germany  and  on  the  southern  slope  of  the  Alps,  was 
inaugurated  by  eruptions  of  quartzose  porphyry,  and  has  terminated  in  the  Alps  by 
those  of  augitic  porphyry.  The  intermediate  epoch  has  been  distinguished  by  rocks 
intermediate  in  composition.  The  mode  of  succession  of  the  different  orders  is  more 
distinct  than  with  granitic,  but  less  so  than  with  volcanic  rocks.  Melaphyr  and  por- 
phyrite interchange  frequently  ;  but,  at  many  places,  the  former  appears  to  have  pre- 
ceded the  latter  in  age,  in  a  similar  way  as  andesite  preceded  trachyte.  They  form 
together  one  epoch,  which,  in  the  commencement,  was  occasionally  interrupted  by 
an  outbreak  of  quartzose  porphyry. — The  laws  of  the  periodical  succession  of  the  five 
orders  of  volcanic  rocks  have  been  developed  in  the  foregoing  pages.  Their  epochs 
are  much  more  distinctly  separated  than  those  of  the  ancient  rocks,  but  the  mode  of 
their  succession  is  more  complicated.  With  granitic  rocks,  silica  as  a  component  part 
decreases  with  the  age ;  with  porphyritic  rocks  the  same  is  true  in  a  broad  sense — the 
precedence  in  age  of  melaphyr  and  porphyrite  forming  the  only  conspicuous  devia- 
tion ;  while  in  reference  to  the  volcanic  era,  no  rule  at  all  may  be  discovered  at  first 
sight.  Considering,  however,  the  predominant  rocks  of  that  era,  which  are  propylite, 

(80) 


OF   VOLCANIC   ROCKS.  43 

andesite,  and  basalt,  we  perceive  that  the  former  two  were  the  precursors  of  the  latter, 
containing  at  the  same  time  silica  in  a  proportion  superior  to  basalt.  The  singular 
phenomenon  of  trachyte  and  rhyolite  being  ejected  at  a  time  intermediate  between 
the  epochs  of  those  rocks,  is  a  notable  deviation  from  the  mode  of  succession  peculiar 
to  granitic  and  porphyritic  rocks.  We  will  try  to  consider  its  probable  causes  in  another 
chapter. 

Correlation  of  Eruptive  Rocks  in  regard  to  their  Geographical  Distribution. 

Another  point  of  view  offering  in  the  contemplation  of  eruptive  rocks,  and  which 
has  a  close  bearing  to  their  natural  system,  is  the  correlation  among  them  in  regard  to 
their  geographical  distribution.  This  is,  however,  a  vast  subject,  and  volumes  might 
be  written  in  collecting  evidence  for  final  argument  of  general  value.  At  this  place 
we  can  give  it  only  a  cursory  notice.  We  will  consider,  first,  what  is  the  mode  of 
distribution  peculiar  to  each  class  of  rocks,  and  then  trace  the  correlations  perceptible 
among  them  in  regard  to  their  different  modes  of  distribution. 

Granitic  rocks  are  scattered  widely  over  the  globe.  Wherever  its  surface  is 
composed  of  ancient  sedimentary  rocks,  and  these  give  evidence  of  disturbances  of 
some  intensity,  by  the  plication  of  their  strata,  we  may  be  almost  certain  to  find  granite 
entering  into  the  geological  composition  to  some  extent.  In  the  diversified  structure 
of  the  European  continent,  geological  maps  show  the  existence  of  granite  in  nearly 
every  prominent  mountain-range.  Considering  among  them  the  range  of  the  Alps  and 
Carpathians,  we  find  .granite  scattered  over  its  whole  extent,  from  Savoy  to  Transyl- 
vania, particularly  on  the  southern  slope.  It  forms  some  prominent  summits,  but 
occurs  also  in  subordinate  positions.  The  geological  relations  of  several  of  these 
places  have  been  examined,  and  careful  investigations  made  of  the  mineral  and  chem- 
ical composition  of  the  rocks.  They  have  resulted  in  proving  an  individuality  of 
granite  such  as  is  peculiar  to  no  other  kind  of  eruptive  rocks  ;  different  granitic 
masses  are  of  different  age,  and  exhibit  a  corresponding  diversity  in  regard  to  the 
composition  of  the  predominating  rocks.  Sometimes,  it  is  true,  several  neighboring 
masses  are  similar  in  respect  to  their  petrographical  character,  and  bear  evidence  of 
being  nearly  of  the  same  age  (for  instance  those  of  the  Adamello,  the  Cirna  d'  Asta, 
and  Brixen),  but  others,  next  adjoining,  will  be  found  differing  from  them  in  nature 
and  in  age.  The  length  of  geological  time  during  which  they  have  been  ejected,  has 
never  been  established  ;  but  the  period  appears  to  have  been  one  of  immense  dura- 
tion. A  similar  individuality  as  to  age  and  mineral  character  may  be  noticed  in 
respect  to  the  granite  of  other  mountain  ranges  on  the  European  continent.  Alto- 
gether, the  mode  of  distribution  of  the  ancient  granitic  rocks,  as  far  as  they  enter  into 
the  structure  of  the  surface,  may  be  said  to  be  in  numerous  small  districts,  which  are 
independent  of  each  other  in  regard  to  their  epoch  of  ejection  and  petrographical  char- 
acter. With  reference  to  the  latter,  each  separate  district  shows  a  great  preponder- 
ance of  rocks  belonging  to  one  of  the  families  of  the  granitic  order,  which  are  usually 
accompanied  by  syenite  in  smaller  proportion,  and  by  some  subordinate  eruptions  of  dior- 
ite  and  diabase.  These  districts  are  principally  scattered  along  the  present  lines  of  eleva- 

(81) 


44  RICIITHOFEN — NATURAL    SYSTEM 

tion.  But  this  may  be  duo  in  part  to  their  concealment,  in  the  spaces  between  those 
lines,  by  sedimentary  rocks.  More  signs  of  the  separation  of  distinct  "regions  of  eruptive 
action,"  as  they  may  be  called,  are  exhibited  in  the  porphyritic  era.  Eruptive  activ- 
ity has  been  intense  within  them,  but  appears  not  to  have  spread  beyond  certain  boun- 
daries. Each  of  these  regions  embraces  a  number  of  the  former  granitic  districts, 
while  it  leaves  others  excluded.  Some  resemblance  with  the  peculiar  features  of  the 
granitic  era  is  afforded,  inasmuch  as  each  porphyritic  region  has  been  independent 
from  others  in  regard  to  the  epftch  of  its  eruptive  activity.  We  mentioned  before 
that  one  of  the  porphyritic  regions  comprises  the  middle  part  of  Germany,  while 
another  stretches  along  the  southern  slope  of  the  Alps  and  Carpathians. — If  we 
proceed  to  the  volcanic  era,  it  presents  to  us  the  reverse  of  the  individuality 
peculiar  to  granitic  districts,  in  the  wonderful  unity  exhibited  in  regard  to  time 
and  space  over  the  whole  area  of  extensive  belts.  In  reference  to  unity  of  time 
alone,  we  might  call  the  greater  part  of  the  continent  of  Europe,  and  even  the 
entire  surface  of  the  globe,  one  great  region  of  eruptive  action,  during  the  vol- 
canic era,  since  the  first  emission  of  rocky  matter  has  been  nearly  cotemporancous 
in  widely  separated  countries,  while  its  culminating  epochs  have  probably  varied  but 
little  in  them  separately,  and  the  rocks  have  been  ejected  everywhere  in  a  similar 
order  of  succession.  In  regard  to  local  distribution,  however,  we  have  to  distinguish 
certain  belts,  far  exceeding  in  area  the  porphyritic  regions.  Each  of  them  extends 
over  a  number  of  preexisting  mountain  ranges,  and  the  eruptions  in  each  have  fol- 
lowed, in  their  distribution,  chiefly  the  lines  of  former  elevation  and  ancient  sea- 
coasts.  But  there  is  unmistakably  to  be  recognized  a  tendency  of  the  agencies  which 
caused  the  eruptions,  to  connect  these  separated  ancient  belts  of  elevation  ;  either  lon- 
gitudinally, when  ranges  superior  in  extent  to  the  preceding  ones  would  be  formed,  as 
appears  to  have  been  the  case  in  the  Andes  ;  or,  as  it  were,  in  a  lateral  sense,  when 
the  connection  of  neighboring  mountain  ranges  into  table  lands  would  be  either 
initiated  or  promoted.  One  of  these  belts,  consisting  of  parts  which  had  previously 
been  disconnected,  may  be  traced  from  Armenia  to  the  Rhine,  though  I  will  try  to 
show  in  the  sequel  that  it  is  only  a  part  of  another  belt  which  is  of  far  greater  extent. 
We  mentioned  before,  that  porphyritic  rocks  are  encountered  chiefly  in  those 
places  where  granitic  rocks  had  preceded  them.  As  regards  the  volcanic  belts,  eruptive 
activity  has  been  particularly  violent  in  certain  portions  of  them.  It  is  worthy  of 
note  that,  wherever  this  has  been  the  case,  either  granite  or  both  granitic  and  por- 
phyritic rocks  had  been  ejected  before.  This  fact  leads  us  to  consider  the  correlation  of 
the  three  classes  of  eruptive  rocks  in  reference  to  their  peculiar  modes  of  distribution. 
Little  information  exists  in  regard  to  this  subject.  Only  one  instance16  shall  be  related, 
which  is  highly  suggestive  for  the  existence  of  such  a  correlation,  though  it  is  of  slight 
value  as  long  as  it  is  not  corroborated  by  corresponding  facts  observed  in  other  coun- 
tries. A  survey,  on  a  geological  map  of  the  Alps  and  Carpathians,  of  the  southern 
boundary-line  of  those  highly  metamorphosed  formations  which  preceded  the  Trias  in 


16   Referred  to  in  Kiclithofcn,   Geognostisclie  Bcclireibimg  der  Umgcgcml  von  Predazzo,  Sunct  Cassian  und 
Seisser  Alp  in  Sud-Tyrol :  Gotha,  1860. 

(82) 


OF    VOLCANIC    ROCKS.  45 

age,  and  chiefly  compose  the  central  portion  of  the  Alps,  shows  that  it  is  directed  from 
west  to  east  in  Lombardy,  but  in  the  vicinity  of  Lugano  bends  suddenly  to  the  north- 
east, then  turns  as  abruptly  back  to  its  former  direction  from  west  to  east.  After 
having  passed  the  granitic  mass  of  the  Adamello,  the  same  change  is  repeated  on  a 
grander  scale.  The  boundary  is  turned  again  in  a  perfectly  straight  line  to  the  north- 
east, and  then  resumes  its  former  course,  which  it  follows  in  an  equally  direct  man- 
ner, and  in  which  it  continues,  exceedingly  distinct  at  first — less  distinct,  by  the 
encroachment  on  it  of  more  recent  formations,  farther  east — until  it  turns  a  third  time 
to  the  northeast  at  the  sudden  termination  of  the  Alps  near  Vienna,  and  continues  in 
this  direction  for  a  long  distance.  Finally  it  re-assumes,  in  the  Carpathians,  a  similar 
course  to  that  which  it  had  on  the  southern  slope  of  the  Alps.  Three  very  distinct 
reentering  angles  are  thereby  formed.  The  first  of  them  encloses  the  country  of  Lu- 
gano ;  the  second  comprises  the  vicinity  of  Predazzo  and  Fassa  in  southeastern  Tyrol, 
and  of  Belluno  and  Viccnza  in  Venetia  ;  while  the  third,  which  is  by  far  the  most 
extensive,  comprehends  all  northwestern  Hungary.  Each  of  them  has  been  a  center 
of  eruptive  activity,  commencing  with  the  granitic,  and  continuing  through  the  por- 
phyritic  down  to  the  volcanic  eras,  and  all  three  are  among  the  most  classical  coun- 
tries for  the  study  of  eruptive  rocks.  There  is,  however,  a  conspicuous  difference  in 
the  mode  of  manifestation  of  the  eruptive  activity  in  each  of  the  three  eras.  Little 
connection  exists  apparently  between  the  granitic  masses  of  the  three  countries.  They 
are  portions  of  the  generally  scattered  granitic  outbreaks,  and  differ  among  themselves 
probably  as  much  in  age  as  they  do  in  regard  to  the  nature  of  their  rocks.  In  the 
porphyritic  era,  eruptive  activity  was  contemporaneous  in  the  three  localities,  but 
scarcely  extended  beyond  them.  In  the  volcanic  era,  when  the  southern  slope  of  the 
Alps  and  Carpathians  formed  only  a  portion  of  a  much  more  extensive  belt,  the 
countries  adjoining  those  three  places  were  chiefly  distinguished  by  the  intensity  of 
eruptive  activity. 

Great  as  have  been  the  interruptions  between  the  different  eras,  the  continu- 
ance of  the  selection  of  those  three  nooks  at  the  foot  of  a  prominent  mountain  range 
for  the  manifestation  of  subterranean  energy,  from  Palaeozoic  down  to  modern  time, 
is  evident.  Similar  instances,  though  less  striking,  might  be  mentioned  from  other 
parts  of  Europe,  such  as  the  porphyritic  region  of  middle  Germany.  Reverting  to 
other  parts  of  the  globe,  it  appears  to  be  a  general  experience,  though  it  is  far  from 
being  absolutely  proved,  that  all  the  principal  accumulations  of  volcanic  rocks  are 
encountered  in  the  neighborhood  or  immediate  vicinity  of  granitic  masses.  These  are 
scattered  over  areas  where  no  volcanic  rocks  occur  ;  but  the  distribution  of  the  latter 
within  any  of  the  volcanic  belts  appears  to  have  been  dependent,  in  a  great  measure, 
upon  the  vicinity  of  the  channels  which  had  in  preceding  time  afforded  vent  to  granite. 

The  general  law  deduceable  from  these  relations  is  this  :  that,  with  the  growing 
thickness  of  the  earth's  crust,  the  systems  of  fractures  which  were  formed  in  it  at  cer- 
tain epochs,  and  partly  gave  vent  to  the  emission  of  rocky  matter,  increased  in  depth 
as  well  as  in  length,  and  were  more  and  more  concentrated  to  definite  portions  of  the 
crust,  which  are  recognizable  upon  the  earth's  surface  by  the  partly  coincident  areas  of 

K  (83) 


46  RICHTHOFEN NATURAL   SYSTEM 

eruption  ;  that  further,  simultaneously  with  the  increase  in  extent,  each  area  manifested 
a  growing  complexity  in  regard  to  the  distribution  of  eruptive  activity  within  it,  and 
that  this  distribution  depended  in  a  great  measure  upon  that  of  the  outlets  of  more 
ancient  eruptive  matter.  The  first  recognizable  stage  of  this  development,  which, 
however,  was  probably  a  far  advanced  one,  is  the  individualization  of  those  nu- 
merous districts  of  fractures,  the  narrow  limits  of  which  are  made  manifest  by  the 
mode  of  occurrence  of  Azoic  and  Palaeozoic  granite.  A  growing  development  from  that 
stage,  in  the  different  directions  mentioned,  is  conspicuous  in  the  porphyritic  era  in 
Europe  ;  when,  besides  the  greater  extent  of  the  regions  in  which  subterranean  agencies 
manifested  themselves  by  the  fracturing  of  the  crust,  every  such  area  had  become 
more  definite  concerning  its  boundaries  towards  those  which  were  not  fractured  during 
the  same  era.  On  the  other  hand,  however,  each  porphyritic  region  was  more  complex 
than  the  granitic  districts  had  been,  inasmuch  as  the  former  were  composed  of  certain 
areas  of  greatest  activity,  which,  as  we  had  occasion  to  remark,  appear  to  have  been 
chiefly  dependent  upon  the  distribution  of  the  granitic  districts  enclosed  within  each 
porphyritic  region,  while  intermediate  portions  were  contemporaneously  affected  by 
disturbances  merely,  but  not  by  any  eruptions.  A  more  advanced  stage  in  this  gradual 
development  seems  to  be  exhibited  by  the  Jurassic  granite  of  the  Andes.  The  belt 
distinguished  by  its  eruption  appears  to  have  been  distinctly  bounded,  and  the  area 
over  which  contemporaneous  manifestations  of  like  character  took  place,  to  have  been 
even  more  extensive  than  the  porphyritic  regions  of  Europe.  All  these  features,  how- 
ever, are  conspicuous  on  a  much  grander  scale  in  the  volcanic  era.  The  growing 
definiteness  of  the  boundaries  of  the  volcanic  belts  towards  large  areas  which  were 
entirely  free  of  eruptive  activity,  the  increasing  complexity  of  their  interior  arrangement, 
and  its  dependency  upon  preexisting  lines  of  elevation,  and  particularly  on  the 
distribution  of  granitic  and  porphyritic  rocks,  are  evident  from  what  has  been  said  before. 
We  may  even  trace  a  development  from  the  andesitic  to  the  basaltic  epoch.  It  is 
known  how  far  superior  in  extent  is  the  range  of  basaltic  outbreaks  to  the  area  occupied 
by  other  volcanic  rocks.  If  considered  with  attention,  it  will  be  found  that  there  is  a 
tendency  in  the  former  to  connect  within  each  belt  the  subordinate  ranges  of  the  latter, 
longitudinally  as  well  as  laterally.  The  only  connection,  for  instance,  between  the 
volcanic  districts  of  Hungary  and  those  on  the  lower  Rhine,  is  occasioned  by  the  chain 
of  isolated  basaltic  hills  which  extends  through  the  central  part  of  Germany. 


ON  THE  ORIGIN  OF  VOLCANIC  ROCKS. 

The  facts  established  in  the  foregoing  chapters,  and  the  inferences  drawn  there- 
from, may  assist  us  in  considering  some  questions  of  wider  bearing.  We  have  seen  that 
the  volcanic  rocks  of  several,  and  probably  of  all  parts  of  the  globe,  are  connected  by 
simple  and  definite  relations,  which  together  comprehend  the  main  features  of  their 
natural  system ;  and  we  found  that  correlations  of  a  similar  nature  ally  the  volcanic 
with  all  the  ancient  eruptive  rocks,  justifying,  to  some  extent,  the  supposition  that  the 
eruptive  rocks  of  all  ages  and  places  form  one  harmonious  whole,  and  that  we  may  be 
(84) 


OF   VOLCANIC    ROCKS.  47 

able  to  discover  the  laws  of  the  natural  system  embracing  their  totality.  In  order  to 
arrive  at  a  more  perfect  understanding  of  this  system,  we  must  now  attempt  to  exam- 
ine into  the  causes  of  those  mutual  relations.  It  would  naturally  occur  to  us  that  they 
would  be  implied  in  the  circumstances  which  attended  the  generation  of  the  eruptive 
rocks,  and  in  the  conditions  in  which  they  have  been  before  arriving  at  the  places  they 
occupy  at  present.  We  have,  therefore,  to  investigate  the  following  questions  :  What 
was  the  nature  of  volcanic  rocks  before  they  arrived  at  the  places  which  we  now 
see  them  occupying?  Where  did  they  originate  ?  By  what  agencies  did  rocks  con- 
nected in  widely  separated  places  by  simple  and  definite  relations  come  to  their 
positions  among  others  which  bear  no  such  relations  either  to  them  or  among  each 
other  ?  The  importance  of  these  questions  for  our  subject,  the  -attention  which  they 
have  attracted  through  the  whole  history  of  geological  science,  and  the  great  diversity 
of  opinion  prevailing  in  regard  to  them,  will  make  it  necessary  to  treat  them  more 
fully  than  might  otherwise  appear  consistent  with  the  objects  of  this  paper. 

Volcanic  action  and  massive  eruptions,  notwithstanding  the  similarity  of  the 
material  produced  by  both,  would  appear,  from  the  most  cursory  review  of  the 
phenomena  connected  with  either  of  them,  to  differ  to  some  extent,  not  only  in  regard 
to  the  causes  to  which  they  owe  their  origin,  but  also  in  regard  to  the  position  the 
matter  occupied  before  its  ejection.  We  have  for  this  reason  to  keep  again  distinctly 
separated  these  two  modes  of  manifestation  of  subterranean  energy.  In  regard  to 
massive  eruptions,  which  will  first  occupy  our  attention,  we  can  scarcely  draw  up  any 
argumentation  without  enlarging  on  the  entire  range  of  eruptive  rocks,  and  extending, 
at  least  partially,  our  views  to  them.17 

1.     On  the    Origin  of  Massive   Eruptions. 

In  order  to  establish  some  positive  premises  available  for  drawing  conclusions 
in  regard  to  the  origin  of  the  massive  eruptions  of  volcanic  rocks,  we  reiterate  the 
following  facts,  of  which  mention  has  partly  been  made  in  the  foregoing  pages  : 
1st.  The  eruptive  (including  the  volcanic)  rocks  offer  a  great  diversity  of  chemical 
composition  ;  but  all  the  compounds  represented  by  them  are  mutually  connect- 
ed by  simple  and  definite  arithmetical  relations  as  regards  the  figures  which 

17  The  following  considerations  are  given  notw  ithout  some  hesitation,  partly  on  account  of  the  uncertain  ground  on 
which  they  have  to  move,  and  partly  because  some  of  their  main  features  are,  of  necessity,  only  well  known  theories  repro- 
duced, though,  perhaps,  under  a  somewhat  different  form.  Yet,  the  establishment  of  the  relations  detailed  in  the  preceding 
chapters,  and  other  observations  made  of  late  years,  may  allow  us  to  arrive  at  more  satisfactory  conclusions  in  regard  to 
some  weighty  problems  than  could  be  done  before,  or,  at  least,  to  determine  more  precisely  the  only  direction  in  which  we 
have  to  look  for  their  solution.  It  should  be  borne  in  mind  that  among  the  theories  recently  proposed  upon  the  subjects 
specified  above,  there  is  not  one  which  has  not  already  had  its  prototype  in  the  phantasmagorias  of  the  time  of  the  dawn  of 
geological  science,  and  that  it  is  these  which  have  been  constantly  reproduced,  enlarged,  diversified,  remodeled  according  to 
the  advance  of  science,  and  supported  by  continuous  accumulation  of  evidence.  Propositions  which  had  been  accepted  as 
being  beyond  the  necessity  of  proof,  and  which  are  still  occasionally  reproduced  as  axioms  in  popular  works  on  geology,  have 
been  weakened,  and  not  unfreqnently  overthrown,  when  facts  newly  revealed  would  withdraw  their  chief  supports,  but  have, 
after  some  time,  revived  under  new  forms.  In  treating  on  a  topic  where  the  degree  in  which  the  results  of  our  speculation 
appear  satisfactory  to  us  depends  upon  the  degree  of  probability  which  we  think  we  see  in  the  theories  arrived  at,  and  of 
their  faculty  of  explaining  observed  facts,  and  where  we  are  in  constant  danger  of  making  incorrect  deductions  from  imper- 
fect premises,  not  enough  can  be  done  in  the  way  of  weighing  the  evidence  by  which  different  doctrines  are  supported  ;  and 
this  is  particularly  necessary  in  reference  to  those  theories  which  we  are  too  much  accustomed  to  consider  as  matters  of  fact, 
upon  which  further  conclusions  mav  be  safdy  built. 

(85) 


48  RICHTHOFEN NATURAL   SYSTEM 

express  the  value  of  the  different  ingredients  entering  into  their  composition.  The 
law  of  Bunsen,  in  which  are  embodied  these  relations,  though  true  for  all  eruptive 
rocks,  has  never  been  applicable  to  those  of  sedimentary  origin,  nor  to  the  met- 
amorphic  sedimentary  rocks.  2d.  The  order  in  which  massive  eruptions  of  vol- 
canic rocks  have  taken  place  in  different  countries,  is  by  no  means  conformable  to 
a  regular  succession  of  gradations  in  chemical  or  mineral  composition,  but  shows 
the  existence  of  several  distinct  groups,  each  of  which  comprises,  chemically,  a  certain 
portion  of  the  entire  range  of  compounds  intermediate  between  the  two  extreme  types, 
and  has,  besides,  its  own  peculiarities  of  texture  and  mineral  composition.  It  is  for 
the  latter  reason  that  rocks,  belonging  geologically  to  two  different  groups,  may  be 
identical  as  chemical  compounds,  and  yet  differ  petrographically.  We  may  repeat 
here  that  these  groups,  wherever  their  mutual  relations  have  been  made  an  object  of 
study,  occur  in  the  same  order  of  time,  and  that  the  rocks  belonging  to  each  of  them 
present  generally  a  great  similarity  in  character  wherever  they  are  encountered. 
3d.  The  massive  eruptions  of  volcanic  rocks  are  distributed  over  the  globe  in  certain 
belts,  differing  in  extent  and  width  as  well  as  in  direction. 

There  are,  among  those  of  a  general  bearing,  two  manifest  conclusions  at  which 
we  may  arrive  by  the  aid  of  these  facts.  The  first  relates  to  the  origin  of  the  matter 
ejected.  It  may  be  said  that  equality  of  physical  and  chemical  properties,  and  of  the 
mode  of  occurrence,  will  commonly  involve  equality  of  origin.  We  have,  therefore, 
to  infer,  that  the  source  from  which  volcanic  rocks  have  derived  their  origin  has 
been  similar  in  nature  in  every  locality,  that  the  definite  numerical  relations  existing 
in  the  chemical  composition  of  the  matter  ejected  must  exist  similarly  at  that  source, 
and  that  they  must  there  pervade  matter  equally  in  different  parts  of  the  globe  ;  and 
we  may  further  add,  that  at  the  same  source  the  different  kinds  of  matter  which  cor- 
respond to  the  different  passages  in  composition  among  volcanic  rocks,  must  be  ar- 
ranged, at  every  locality  alike,  in  a  definite  order  of  position  in  reference  to  the 
distance  from  the  center  of  the  earth,  since  such  relation  in  space  can  alone  explain 
the  definite  order  of  succession  in  time  in  which  those  rocks  have  been  ejected.  If 
we  consider  the  relations  existing  between  volcanic  and  ancient  eruptive  rocks,  there 
can  be  no  doubt  that  all  of  these  have  derived  their  origin  from  that  same  source,  and 
that  it  is  of  general  distribution  under  the  surface  of  the  globe.  The  second  general 
conclusion  relates  to  the  cause  and  mode  of  ejection.  As  like  effects  imply  like 
causes,  and  the  similarity  of  the  phenomena  connected  with  the  massive  eruptions  of 
volcanic  rocks  is  conspicuous,  we  may  infer  that  the  agencies  to  which  they  have  been 
due,  were,  in  the  main,  similar  in  all  cases.  However  accidental  and  local  circum- 
stances may  have  caused  minor  differences,  the  prominent  features  in  the  mode  of 
geological  occurrence  and  geographical  distribution  cannot  be  due  to  them.  In  how 
far  we  may  be  justified  to  ascribe  the  ejection  of  the  granitic  and  porphyritic  rocks  to 
similar  agencies,  will  depend  upon  the  degree  of  similarity  of  their  part  in  the  struct- 
ure of  the  surface  of  the  globe  with  that  of  the  volcanic  rocks.  The  conclusion  must 
be  drawn  from  those  correlations  which  have  already  been  mentioned,  that  the  funda- 
mental causes  of  all  eruptive  activity  are  alike,  and  implied  in  general  planetary  pro- 
cesses which  are  closely  connected  with  the  evolution  of  the  globe.  These  general 


OF   VOLCANIC    ROCKS.  49 

conclusions  appear  to  be  the  only  ones  which  are  fully  justified.  If  we  venture  to 
inquire  into  the  nature  of  the  agencies  which  caused  eruptive  activity,  and  to  explore 
their  fundamental  causes,  we  have  to  transgress  the  limits  of  legitimate  speculation 
and  enter  the  realm  of  hypothesis,  the  only  way  of  proceeding  in  which  consists  in 
weighing  probabilities.  Further  experience,  it  may  be  hoped,  will  e.xtend  the  limits 
of  induction,  in  the  same  measure  as  it  will  furnish  increased  evidence. 

I  will  attempt,  in  the  first  place,  to  consider,  what  must  be  the  nature,  and 
what  the  position  of  the  source  at  which  the  volcanic  and  all  eruptive  rocks  have 
originated,  in  order  that  their  physical  and  chemical  properties  and  their  mutual  rela- 
tions may  be  explained,  and  then  to  examine  what  have  been  the  agencies  that  have 
had,  in  all  probability,  the  most  immediate  influence  upon  the  facts  connected  with 
the  mode  of  distribution  and  succession  of  eruptive  rocks  in  general.  We  have  to 
start  in  these  considerations  by  a  well-known  argument. 

If  no  changes  had  ever  taken  place  on  the  surface  of  the  globe,  and  no  sedi- 
ments had  ever  been  deposited  on  it,  but  that  state  was  still  preserved  which  the 
globe  must  have  exhibited  when  the  substance  of  the  rocks  was  liquid  on  its  surface, 
then  the  matter  nearest  to  this  would  probably  be  of  a  nearly  uniform  chemical  com- 
position, and,  consequently,  of  a  uniform  specific  gravity.  If  we  imagine  the  globe  to 
consist  of  concentric  layers,  it  is  exceedingly  probable,  from  physical  laws,  that  then  the 
substance  of  each  successive  layer  would  be,  too,  of  a  nearly  uniform  composition 
throughout  its  extent.  Proceeding,  however,  from  those  layers  nearest  to  the  surface 
to  those  at  greater  distance  from  it,  the  specific  gravity  of  the  matter  composing  them 
severally  would  necessarily  increase  gradually  towards  the  interior.  This  would  be 
effected  by  the  tendency  of  the  heavier  elements  to  predominate  as  the  depth  became 
greater,  while  the  lighter  would  increase  in  as  gradual  a  ratio  towards  the  surface. 
If  the  layers  were  infinite  in  number,  then  the  passage,  in  chemical  composition  as  well 
as  in  specific  gravity,  from  the  surface  towards  the  center  of  the  earth  would  be  one  of 
infinite  gradations.  This  condition  is  eminently  the  most  probable,  if  physical  laws 
are  taken  into  consideration.  If  the  crust  was  then  allowed  to  solidify,  to  any  depth 
required,  the  same  interior  arrangements  would  continue  to  exist  unaltered,  regardless 
of  any  changes  on  the  surface  That  a  condition  similar  to  this  actually  exists  at  the 
present  time,  is  rendered  evident  by  the  well-known  fact,  that  the  specific  gravity  of 
the  mass  of  the  globe  greatly  exceeds  that  of  the  average  of  the  matter  which  com- 
poses its  exterior  crust.  Eruptive  rocks  have  been  carried  to  the  surface  from  places 
beneath  it.  The  depth  from  which  their  material  has  been  derived,  the  way  in  which 
it  was  rendered  liquid,  and  the  cause  and  mode  of  its  ejection,  these  are  the  principal 
points  of  conjecture  in  regard  to  them.  The  most  probable  place  of  its  derivation 
are  those  imaginary  layers  which  are  beneath  the  theater  of  external  changes,  and  still 
occupy  their  primeval  position.  This  appears  to  be  manifest  from  the  two  facts  to 
which  we  have  constantly  to  refer  :  the  ejection  of  identical  chemical  compounds  in  all 
countries  and  ages,  and  the  exhibition  by  eruptive  rocks,  in  each  country,  as  well  as 
over  the  entire  globe,  of  a  series  of  compounds  the  specific  gravity  of  which  increases 
in  inverse  ratio  with  the  amount  of  silica.  The  suggestion  that  eruptive  rocks,  in 
virtue  of  these  facts,  represent  the  arrangement  of  matter  in  the  interior  of  the  globe, 

(87) 


50  RICUTHOFEN —  NATURAL   SYSTEM 

those  which  are  rich  in  silica  and  of  little  specific  gravity  being  derived  from  places 
nearer  to  the  surface  than  those  in  which  a  smaller  proportion  of  silica  is  attended  by 
a  higher  specific  gravity,  was  first  distinctly  made  by  Sartorius  vou  Waltershausen, 
who  attempted  to  prove  this  theory  by  mathematical  calculation,  and  has  at  least  suc- 
ceeded in  demonstrating  its  adaptation  to  the  nature  of  eruptive  rocks.  The  assump- 
tion, hypothetical  as  it  may  still  appear,  has  an  eminent  degree  of  probability,  and  is  of 
the  highest  importance  for  the.geology  of  these  rocks,  for  the  reason  that  it  is  capable 
of  explaining  numerous  observed  facts  which  have  failed  to  be  satisfactorily  explained 
in  any  other  way. 

We  may  still  draw  a  line  of  negative  argument  in  corroboration  of  this  theory. 
It  is  perfectly  clear  that  the  source  of  eruptive  rocks  must  either  have  been  below  the 
lowest  depth  at  which  rocks  of  sedimentary  origin  occur,  or  above  it.  We  found  that 
the  former  assumption  gave  a  satisfactory  explanation  of  prominent  facts,  and  it  is 
just  as  conspicuous  that  the  second  fails  to  give  any  explanation  at  all.  If  eruptive 
rocks  had  had  their  original  seat  within  the  crust  of  sedimentary  rocks,  and  had  been 
generated  from  their  substance,  they  must  be  analogous  to  them  in  chemical  compo- 
sition, that  is  to  say,  they  would  vary  in  this  respect  within  very  wide  and  complex 
limits  in  any  single  country,  and  we  should  encounter  no  lesser  differences  when  com- 
paring the  totality  of  eruptive  rocks  in  one  part  of  the  globe  with  that  in  other  parts. 
The  fact  that  no  arithmetical  relations  can  be  recognized  as  connecting  the  various 
sedimentary  rocks  in  their  composition,  would  hold  equally  good  among  eruptive 
rocks,  since  it  is  impossible  to  conceive  a  progress  from  the  indefinite  to  the  definite, 
from  that  which  is  void  of  any  recognizable  relations  in  respect  to  the  composition  of 
matter  to  that  in  which  such  definite  relations  are  plainly  evident,  by  the  mere 
influence  of  such  agencies  as  would  cause  fluidity,  (that  is,  the  combined  action  of 
heat,  pressure,  and  water.)  As  no  agency,  indeed,  is  known  to  which  such  a  result 
may  be  ascribed,  this  argument  removes  beyond  the  limits  of  probability  the 
assumption,  that  the  original  seat  of  eruptive  rocks  has  been  above  the  foundation  of 
the  gradual  accumulation  of  those  rocks  which  have  been  produced  by  external 
changes,  and  therefore  allows  only  the  other  alternative,  that  it  has  been  below  that 
boundary. 

While  the  theory  of  Sartorius  will  explain  the  causes  of  the  prominent  mutual 
relations  existing  among  eruptive  rocks  in  regard  to  their  composition  and  general 
similarity  in  all  countries,  we  have  still  to  trace  those  causes  which  effected  their  ejec- 
tion from  a  deep-seated  original  place  to  the  surface.  This  question  is  more  abstruse 
than  the  foregoing,  inasmuch  as  the  manner  in  which  forces  have  formerly  been  acting 
beneath  the  earth's  crust,  is  a  subject  more  involved,  and  allowing  a  larger  range  of 
hypothetical  explanation,  than  we  met  on  a  field  where  the  observed  properties  of 
matter  offered  a  comparatively  safe  guide.  Complicated  as  the  processes  appear  to 
have  been,  to  the  cooperation  of  which  massive  eruptions  were  due,  it  seems  that 
they  may  severally  be  traced  back  to  one  common  cause  of  a  higher  order,  of  which 
all,  or  most,  of  them  are  but  different  modes  of  manifestation.  Probabilities  accumu- 
late to  point  out  as  this  fundamental  cause,  the  process  of  the  gradual  cooling  of  the 
earth  and  the  solidifying  of  its  crust  toward  its  interior. 

(88) 


OF   VOLCANIC   ROCKS.  51 

Since  the  early  day  of  the  ingenious  speculations  of  Descartes,  this  great  and 
general  cause  has  been  considered  as  the  main  agency  to  which  the  disturbances 
on  the  surface  of  the  globe  are  due  ;  and,  though  having  been  more  or  less  in 
favor  at  different  times,  the  doctrine  has  at  no  time  been  completely  abandoned. 
The  various  aspects  which  it  has  periodically  assumed,  the  latitude  given  to  it 
on  one  side,  and  the  objections  raised  against  it  on  the  other,  mark  one  feature 
of  the  phases  of  the  gradual  progress  of  science.  It  has  been  applied  in  different 
forms  to  explain  the  mode  of  origin  of  ancient  eruptive  rocks  ;  and,  since  Dolo- 
mieu  and,  in  a  more  elaborate  way,  his  pupil  Cordier,  have  assigned  the  same 
original  source  to  volcanic  activity,  the  contraction  of  the  interior  of  the  globe,  by 
radiation  of  its  heat  into  space,  has  been  considered  as  offering  a  sufficient  explana- 
tion for  the  majority  of  the  phenomena  which  are  often  united  under  the  term 
' '  vulcanism."  But,  giving  all  due  consideration  to  the  vast  effects  of  which  it  has 
undoubtedly  been  the  cause,  the  conception  of  the  modus  operandi  of  this  agency 
(contraction)  alone  meets  with  considerable  difficulty.  An  outward  tension  might, 
indeed,  result  in  the  formation  of  fractures  on  elevated  places  ;  and,  supposing  for  a 
moment  that  these  fractures  would  descend  into  regions  where  matter  was  in  a  liquid 
state,  then  the  latter  might  possibly  be  ejected,  and  caused  to  accumulate  on  the  sur- 
face. But  such  would  hardly  be  the  effect  of  an  inward  tension.  It  may,  too,  cause 
the  rending  of  the  crust  and,  possibly,  the  filling  by  liquid  matter  of  those  rents 
which  are  in  the  lowest  places  ;  but  it  does  not  explain  the  extrusion  of  this  matter 
to  the  surface,  nor  the  fact  of  its  particular  accumulation  on  elevated  parts  of  the 
same.  It  would  take  too  much  of  our  space  fully  to  detail  the  numerous  mechanical 
difficulties  which  occur,  if  one  attempts  to  explain  all  the  phenomena  comprised  in  the 
name  "  vulcanism"  by  the  exclusive  assumption  of  the  contraction  of  the  interior  of 
the  globe.  Some  of  the  more  obvious  objections  against  this  theory  will  be  briefly 
mentioned  in  the  course  of  the  following  considerations. 

A  number  of  facts  point  towards  the  existence  of  some  unknown  force  below 
the  earth's  solid  crust,  which  counteracts  in  a  considerable  measure  the  permanent 
subsidence  of  the  latter  by  contraction.  It  is  perfectly  evident  that  the  secular  rising 
of  parts  of  the  surface  of  the  globe  above  the  level  of  the  sea  cannot  be  merely  the 
apparent  effect  of  the  different  degree  of  its  general  subsidence,  as  has  been  maintained 
by  very  distinguished  geologists;  but  that  elevation,  that  is,  the  periodical  increase  of 
the  distance  of  parts  of  the  surface  of  the  globe  from  its  center,  must  be  a  reality. 
Considering  the  amplitudes  of  the  changes  of  level  that  have  taken  place  in  historical 
time,  they  will  be  found  to  sum  up  to  such  figures  as,  if  reduced  altogether  to  subsi- 
dence, would  indicate  a  far  greater  shortening  of  the  radius  of  the  globe  within  that 
time,  than  is  compatible  with  astronomical  calculation.  It  is  true  that  the  retardation 
of  the  rotation  of  the  earth  by  the  tidal  wave  must  counteract,  in  some  measure,  the 
acceleration  caused  by  any  shortening  of  the  radius  of  the  planet ;  but  this  retardation 
is  insignificant  if  compared  with  the  amount  of  the  changes  of  level.  The  reality  of 
elevation  is  forced  more  directly  upon  the  mind,  if  those  cases  are  taken  into  consider- 
ation where  certain  portions  of  continents  are  rising  above  the  level  of  the  sea  at  a 
more  accelerated  rate  than  neighboring  regions,  which  is  of  very  frequent  occurrence 

(89) 


62  RICHTHOFEN NATURAL   SYSTEM 

in  volcanic  countries,  and  conspicuous  in  those  numerous  instances  where  elevation  is, 
or  has  been  in  former  times,  proceeding  more  rapidly  along  the  crests  of  mountain 
ranges  than  at  either  foot  of  them.  Adherents  of  the  theory,  that  all  oscillations  of 
the  earth's  crust  are  only  due  to  its  contraction,  consider  elevation  as  real  in  these 
cases,  and  have  tried  to  explain  it  by  the  assumption,  that  folds  must  be  formed  by  the 
subsidence  of  the  ample  shell  on  the  contracted  nucleus  ;  that  these  folds  would  increase 
in  amplitude  in  consequence  of  a  lateral  pressure  caused  by  further  subsidence,  and 
thus  an  absolute  rise  would  be  effected.  It  is,  indeed,  very  probable  that  this  process 
is  of  vast  importance  in  the  formation  of  mountain  ranges  ;  but  it  cannot  explain  the 
total  amount  of  the  changes  of  level.  If  Fourier's  calculation,  that,  taking  the  present 
loss  of  heat  by  the  globe  as  standard,  the  radius  of  the  latter  should  have  shortened 
seventeen  centimeters  in  twenty-five  centuries,  is  correct,  the  diminution  would  have 
been  1,700  centimeters  in  250,000  years,  and  about  six  hundred  feet  in  one  million 
years.  It  is  manifest  how  utterly  insignificant  would  be  the  corresponding  diminution 
of  the  earth's  circumference,  when  compared  with  the  vast  changes  of  level  which  the 
surface  must  be  supposed  to  have  undergone  during  such  a  length  of  time. 

We  are  bound,  for  these  reasons,  to  consider,  not  alone  the  process  of  elevation 
of  mountain  ranges,  but  also  the  secular  rise  of  extensive  regions,  as  realities  which 
cannot  be  exclusively  explained  by  the  contraction  of  the  globe.  But  if  so,  there  must 
exist  another  force,  the  effects  of  which  oppose  those  of  contraction.  The  united  action 
of  both,  and  the  periodical  prevalence  of  either  of  them,  would  then  be  capable  of 
explaining  the  alternation  of  elevation  and  subsidence  at  every  single  place,  and  the 
contemporaneous  action  of  both  in  neighboring  regions.  This  antagonistic  force  con- 
sists, probably,  in  an  increase  of  volume  attending  the  slow  and  perfect  crystallization 
of  matter,  by  cooling  down,  during  immense  periods,  from  a  viscous  state.  With  a 
great  number  of  bodies,  contraction  by  loss  of  heat  appears  to  continue  to  the  very 
moment  of  solidification,  and  to  increase  during  the  latter  when  no  time  is  allowed  for 
crystallization,  but  to  be  diminished,  and  finally  reversed,  in  the  same  measure  as  oppor- 
tunity is  given  for  a  slow  and  perfect  crystallization.  In  the  special  case  of  rocks 
made  up  of  silicates,  this  must  remain  a  supposition  which  is  not  proved,  but  is  emi- 
nently probable.18 

'8  An  increase  in  volume  by  crystallization  has  been  found,  by  experiment,  to  take  place  in  numerous  instances,  in  fact 
in  most  cases  when  a  perfect  crystallization  from  a  molten  state  has  been  obtained,  provided  that  the  volume  of  the  substance 
experimented  on  could  be  determined  immediately  before  the  act  of  crystallizing.  In  respect  to  those  silicates  which  make 
up  crystalline  rocks  (not  taking  into  account  the  water  entering  into  their  composition),  experiments  could  hitherto  not 
be  made,  because  it  is  impossible,  with  our  present  means,  to  allow  the  viscous  mass  sufficient  time  for  crystallizing.  Yet, 
there  are  some  suggestive  facts  which  have,  in  some  measure,  the  value  of  experiments.  We  mention  among  them  par- 
ticularly one  which  was  observed  by  Ferd.  Zirkel  in  analyzing,  with  the  aid  of  the  microscope,  the  texture  of  the 
minerals  participating  in  the  composition  of  eruptive  rocks.  He  found  that  "glass  cavities"  contain  ordinarily  several 
of  those  vacuities  which  are  also  exhibited  by  the  cavities  filled  with  water,  and  have  been  supposed  by  Sorby 
and  others  to  have  originated  from  the  contraction,  by  cooling,  of  the  enclosed  matter.  If  the  substance  contained 
in  the  glass  cavities  bears  signs  of  an  incipient  crystallization,  by  having  partly  a"lithoid"  texture,  the  vacuities  are 
of  rarer  occurrence,  while  they  are  entirely  wanting  in  the  so-called  stone-cavities,  when  the  lithoid  texture  pervades  the 
whole  mass  filling  the  cavity.  It  is  scarcely  possible  to  make  an  experiment  more  convincive  than  this  natural  occurrence, 
which  is  more  open  to  subtle  observation  and  measurement  than  is  ordinarily  the  ca.=e  with  experiments  on  cognate  subjects. 
It  needs  hardly  to  be  mentioned,  in  connection  with  this  question,  that  those  experiments  which  have  been  made  by  Bishof 

(90) 


OF   VOLCANIC    ROCKS.  53 

We  may  then  distinguish  as  eminently  probable,  the  following  immediate  effects 
of  the  cooling  of  the  globe  :  1 .  Contraction  of  the  liquid  portion  of  the  interior,  by 
cooling  down  to  the  temperature  required  for  solidification  under  the  respective  press- 
ure. 2.  Expansion  by  slow  and  perfect  crystallization.  3.  Contraction  of  the  crys- 
tallized masses  by  further  cooling.  We  may  add  to  these  as  a  secondary  effect  of 
greatly  inferior  importance,  the  flow  of  heat  or  change  of  chthouisothermal  planes,  at 
those  places  where  subsidence  causes  an  accumulation,  or  elevation  an  abrasion  of 
matter  on  the  surface.  This  process,  to  which  a  truly  wonderful  importance  has  been 
ascribed  by  some,  must  be  going  on  continually  and  everywhere.  Being  itself  the 
result  of  motions  of  the  crust,  its  proximate  cause  must  be  the  processes  attending  the 
cooling  of  the  globe,  while  in  its  totality,  it  may  have  a  further,  though  very^insignifi- 
cant,  effect  in  shifting  the  areas  of  elevation  and  subsidence. 

Other  agencies,  with  which  we  are  yet  unacquainted,  may  probably  result  from 
the  same  common  cause.  But  taking  into  consideration  only  those  already  mentioned, 
it  may  appear  difficult  to  form  a  clear  conception  of  the  mode  in  which  they  must 
cooperate,  in  order  not  to  counteract  each  other,  and  thereby  to  result  in  a  general 
movement  of  the  crust  in  one  direction  onty,  but  to  bring  about  that  variety  of 
efl'ects  which  manifests  itself  chiefly  in  the  change  of  elevation  and  subsidence,  and  in 
other  modes  of  dislocation  of  the  crust.  It  is  perfectly  evident,  that  the  aggregate 
effect  of  those  agencies  is  different  under  different  portions  of  the  crust  of  the  globe, 
contraction  prevailing  in  some  parts,  and  expansion  in  others  ;  the  former  causing 
subsidence  and  deposition  of  sedimentary  matter,  the  other  elevation  and  denudation. 
But  if  we  consider  how  gradual  is  the  passage  in  the  state  of  aggregation  from  the  liquid 
interior  of  a  current  of  lava  to  its  solid  crust,  and  if  we  bear  in  mind  how  immensely 
vaster  in  volume  are  those  masses  which  constitute  the  interior  of  the  globe,  and  what 
immensely  longer  time  is  given  for  their  cooling  and  crystallization,  the  conclusion  is 
irresistible,  that  the  passage  in  the  states  of  aggregation  from  solid  to  liquid  must 
extend  there  over  an  immensely  greater  space.  Crystallization  will  very  probably 
take  place  next  to  those  portions  of  the  crust  which  are  already  solid.  If  it  is  attended 
by  an  increase  of  volume,  and  this  increase  produces  tension,  it  is  very  likely  that  the 
next  adjoining  masses,  though  not  yet  crystallized,  will  offer  too  much  resistance  to 
allow  this  tendency  for  expansion  to  find  immediate  relief  by  yielding  to  the  gen- 
eral tendency  to  contraction  which  may  prevail  in  liquid  masses  at  greater  depths. 
It  may  thus  be  explained  why,  in  different  parts  of  the  crust,  a  motion,  independent 
and  in  opposite  directions,  may  result  from  the  two-fold  tension  attending  changes 
of  volume  which  take  place  at  different  distances  beneath  the  surface,  and  which 

and  others  with  a  view  of  ascertaining  the  increase  in  volume  which  different  rocks  undergo  by  melting,  do  not  affect  our 
supposition.  In  the  first  place,  the  molten  state  may  be  quite  different  from  that  in  which  the  same  substance  would  be 
immediately  before  crystallizing,  and  the  passage  from  one  to  the  other  may  be  attended  by  a  decrease  in  volume  which  is 
not  counterbalanced  by  the  subsequent  increase  taking  place  by  the  passage  of  the  substance  into  the  crystallized  state. 
Then,  the  rocks  on  which  the  experiments  were  made  had  been  solidified  under  a  great  pressure,  while,  when  in  a  molten 
state,  they  were  only  exposed  to  the  pressure  of  the  atmosphere.  Finally,  it  has  not  been  ascertained  whether  all  the  water 
which  is  contained  in  the  rocks  does  escape  on  melting  or  not.  If  not,  then  it  will  probably  have  the  effect  of  inflating  the 
molten  mass. 

(91) 


54 


RICHTHOFEN  —  NATURAL   SYSTEM 


would  oppose,  and,  to  a  certain  degree,  destroy  each  other,  if  the  state  of  aggregation 
of  subterranean  matter  allowed  of  a  free  conduct  of  motion. 

These  slow  and  continuous  agencies,  chiefly  those  among  them  which  give  origin 
to  the  process  of  elevation,  must  have  been,  too,  instrumental  in  causing  the  massive 
eruptions  of  rocks.  At  least,  no  other  force  supposed  to  act  beneath  the  surface  can 
account  as  fully  for  certain  facts  connected  with  them,  such  as  the  gradual  changes 
in  the  mode  of  their  geographical  distribution,  or  the  connection  of  the  manifesta- 
tions in  any  certain  system  of  fractures,  or  the  fact  that  ages  of  comparative 
repose  have  been  interrupted  by  paroxysmal  actions  of  great  violence  which  have 
taken  place  during  certain  eras  in  the  history  of  each  separate  country.  The 
latter  circumstance  points  clearly  to  the  assumption,  that  during  the  eras  of  repose, 
or  at  least  their  later  portion,  a  constantly  increasing  amount  of  potential  energy 
must  have  been  accumulated  under  the  crust  of  the  globe  ;  since  the  subterra. 
nean  agencies  called  into  existence  by  the  cooling  of  the  globe  did  of  course  never 
rest,  nor  can  they  be  reasonably  supposed  to  have  been  more  intense  in  the  Tertiary 
than  they  were  in  preceding  periods.  It  is  self-evident  that  the  increase  of  tension 
must  have  been  much  more  considerable  under  areas  of  elevation  than  under  the 
more  extensive  regions  of  subsidence.  In  the  latter  case,  when  a  downward  tendency 
is  caused  by  contraction,  the  weight  of  the  crust  must  come  in  its  aid  ;  the  essence  of 
the  resistance  to  that,  tendency  may  therefore  be  concisely  expressed  as  cohesion  minus 
weight ;  while  weight  added  to  the  cohesion  will  give  approximately  the  resistance  to 
elevation  by  expansion.  The  tension  under  a  crust  of  great  thickness  must  therefore, 
in  the  latter  case,  increase  to  a  stupendous  intensity,  until  it  is  sufficient  to  overcome 
the  resistance,  and  will  then  be  able  to  result  in  such  paroxysms  as  that  by  which 
the  volcanic  era  has  been  inaugurated,  and  the  main  feature  of  which  consists  in  the 
formation  of  fractures  which,  by  their  mode  of  association,  constitute  distinct  systems 
or  belts,  separated .  by  areas  in  which  the  existence  of  fissures  in  the  crust  cannot  be 
recognized  on  the  surface. 

Although  the  processes  suggested  may  furnish  us  the  cause  for  the  prime  condi- 
tion to  the  emission  of  rocky  matter  from  below,  namely,  the  periodical  opening  in  the 
earth's  crust  of  such  fissures  as  widened  with  the  approach  to  the  surface ;  yet 
they  fail  completely  to  explain,  by  themselves  alone,  the  more  immediate  causes  and 
the  mode  of  that  emission.  It  may  be  presumptive  to  extend  speculation  upon  this 
topic  beyond  those  limits  which  we  have  reached  ;  but  a  safer  guide  to  conjecture  on 
the  same  than  had  been  known  at  any  former  time  was  given  during  the  last  few 
years  by  the  experiments  of  Daubr6e,  and  the  microscopic  examinations  of  the  texture 
of  rocks  by  Sorby.  It  has  been  held  quite  generally  till  of  late,  that  the  opening  of 
a  fissure  to  a  liquid  mass  below,  would  be  sufficient  by  itself  to  cause  the  ascending 
of  the  liquid  through  it  to  the  surface  ;19  but  this  supposition  is  utterly  irreconcilable 

19  It  must  here  he  remarked  that  some  of  the  most  eminent  writer?  on  the  subject  of  voleanoes,  chiefly  Pon- 
lett  Scrope,  Prof.  Dana  and  others,  have  suggested  long  ago,  that  the  ascending  of  lava  in  a  volcanic  channel  must  be 
due  to  both  the  fluidity  and  expansion  imparted  to  it  by  the  globular  state  of  the  water  which  finds  ingress  to  the  chan- 
nels of  the  lava  and  enters  into  its  composition.  They  have  anticipated,  by  this  suggestion,  in  some  measure,  the  results  of 

(92). 


OF   VOLCANIC   ROCKS.  65 

with  physical  laws.  Supposing  for  a  moment  that  theory  to  be  correct  which  consid- 
ers all  the  changes  of  level  as  originating  in  contraction  alone,  then  it  is  quite  proba- 
ble that  fissures  would  be  formed  in  the  center  of  the  areas  of  subsidence.  But  as 
they  must  be  closed  near  the  surface,  and  open  in  their  lower  parts,  it  is  difficult  to 
see  in  what  manner  the  liquid  matter  could  ascend  through  them  to  the  surface,  while 
no  better  account  could  be  given  for  the  occurrence  of  eruptive  rocks  on  high  table- 
lands. In  order  to  explain  it,  recourse  has  been  had  to  the  most  arbitrary  assump- 
tions. It  may  even  be  read  in  our  time  in  various  geological  books,  that  the  crust  below 
might  give  way  from  the  overload,  and  the  whole  be  "  plunged "  into  the  semi-fluid 
mass  beneath,  causing  it  to  overflow.  Mountain  ranges  of  a  thousand  miles  in  extent 
have  been  assumed  to  subside  suddenly  upon  the  liquid  mass,  and  to  "  splash  it  out " 
through  fissures.  Leaving  out  of  consideration  these  fantastical  theories,  there  remains 
a  number  of  others,  according  to  which  the  weight  of  portions  of  the  crust  would 
cause  the  ejection  of  liquid  matter.  If  the  substance  composing  the  crust  exceeded 
in  specific  gravity  the  liquid  matter  below,  then  this  mode  of  ejection  would  be  prob- 
able, and  we  should  be  indeed  surprised  that  large  portions  of  our  globe  were  not 
flooded  over  repeatedly  by  molten  masses  from  below.  If  the  specific  gravity  of  the 
crust  and  the  fluid  matter  below  was  the  same,  then  it  would  require  the  most  exten- 
sive fracturing  of  the  crust  and  uplifting  of  its  fragments,  in  order  to  make  the  liquid 
mass  overflow  the  latter.  But  all  that  we  know  in  regard  to  the  subject  goes  to  show 
that  the  masses  below  the  crust  are  of  greater  specific  gravity  than  those  composing 
it.  To  suppose  the  weight  of  the  crust  to  cause  the  protrusion  of  liquid  matter 
through  fissures,  is  therefore  to  suppose  an  action  which  is  mechanically  impossible. 

We  arrive  at  no  more  satisfactory  conclusion  in  regard  to  our  present  problem, 
if,  besides  the  contracting  forces,  we  assume  the  existence,  beneath  the  crust  of  the 
earth,  of  others  which  have  their  origin  in  the  increase  of  volume  by  crystallization, 
and  thus  produce  expansion.  I  attempted  to  show  that  they  furnish  the  most  probable 
agent  to  which  may  be  ascribed  the  opening  of  the  systems  of  fissures,  which  partly 
served  as  the  channels  for  the  extrusion  of  rocky  matter.  These  fissures  would  have 
to  be  open  at  the  surface  and  to  decrease  in  width  below,  because  formed  by  an  out- 
ward tension  on  areas  of  elevation.  It  cannot  be  assumed  that  they  would  descend  to 
any  greater  depth  than  the  lowest  limits  of  solid  rocks ;  they  would,  therefore,  not 
reach  down  to  any  matter  sufficiently  liquid  as  to  be  capable  of  being  forced  up 
through  them.  And  if  it  should  be  able  to  ascend,  then  it  would  solidify  within  the 
fissure,  almost  instantly,  by  loss  of  heat,  and  long  before  reaching  the  surface.  There 
is,  however,  even  a  more  forcible  argument  to  demonstrate  why  it  should  not  have 
been  capable  of  ever  entering  the  fissures.  For,  if  our  supposition  that  the  silicious 
masses  beneath  the  crust  increase  in  volume  by  crystallization  is  correct,  the  relief 
from  pressure  by  the  formation  of  fissures  must  have  the  immediate  effect  of  rapidly  pro- 
experiment  established  by  Daubree.  But  the  supposition  was  only  made  for  the  case  of  lava,  and  not  for  that  of  rocks  which 
were  ejected  without  volcanic  action  proper.  Yet  even  in  regard  to  volcanoes  it  did  only  explain  the  extrusion  of  lava  to 
the  surface  from  a  place  at  a  limited  distance  below  it,  and  failed  to  give  a  clue  to  the  manner  in  which  the  constant  supply 
of  matter  to  those  places  was  kept  up. 

(93) 


56  RICHTHOFEN  —  NATURAL   SYSTEM 

moting  the  crystallization  of  vast  masses  which  had  been  held  in  a  viscous  state  before, 
by  the  existence  of  the  tension  itself.  It  needed,  for  all  these  reasons,  another  agency 
which  would  not  alone  force  up  molten  matter  through  the  fissures,  but  also  cause  it  to 
arrive  at  the  surface  in  that  particular  state  of  aggregation  which  it  has  had,  according 
to  the  observations  of  Sorby.  This  agency  is  indicated,  by  the  experiments  of  Daubre'e, 
to  have  been  water,  the  descending  of  which  into  the  fractures  is  indeed  a  necessary 
consequence  of  their  formation.  There  it  would  convert  the  state  of  aggregation  of 
the  masses  surrounding  the  lowest  parts  of  the  fracture  into  that  called  by  Daubree 
"  aqueous  fusion,"  which  appears,  indeed,  to  have  been  the  state  in  which  all  eruptive 
rocks  have  been  immediately  before  their  consolidation.  The  process  of  aqueous  fusion, 
as  has  been  shown  by  the  same  eminent  geologist,  is  attended  by  a  very  considerable 
increase  in  volume  of  the  masses  affected.  It  would,  therefore,  give  rise  to  processes 
totally  different  from  those  which  had  preceded.  For  this  expansion  would  imme- 
diately cause  a  motion  of  the  masses  rendered  liquid,  in  the  direction  of  least  resist- 
ance, that  is,  upwards  in  the  fissure,  and  would,  if  continued  for  a  sufficient  time, 
make  the  same  overflow  on  the  surface  of  the  crust,  even  if  unassisted  by  other  eject- 
ing agents,  such  as  the  vapor  of  water.20 

We  may  carry  these  deductions  still  further,  if  we  revert  to  our  previous  con- 
clusion, that  the  relief  from  pressure  by  the  fracturing  of  the  crust  would  have  caused  the 
crystallization  of  masses  below  it,  which  had  been  held  before  in  a  viscous  state  by  the 
tension  itself.  This  process  would  extend  in  depth  as  well  as  laterally,  and  gradually 
affect  the  viscid  masses  beneath  an  entire  belt  of  fissures.  It  would  have  had  again  to  be 
attended  by  an  increase  of  volume.  But  those  crystallizing  masses  not  being  in  a  state 
of  aqueous  fusion,  the  resistance  could  in  this  case  not  be  overcome  by  the  extrusion 
of  that  portion  of  them  by  which  they  were  increased  in  volume,  and  the  effect  would 
be,  as  in  the  case  first  mentioned,  accumulation  of  potential  energy.  I  will  attempt  to 

20  If  we  consider  the  geological  features  of  all  large  accumulations  of  eruptive  rocks,  such  as  the  great  amlesitic 
ranges  of  Hungary,  or  the  quartzose  porphyry  composing  a  plateau  of  great  dimensions  in  southern  Tyrol,  or  those  granitic 
masses  which,  by  overlying  the  edges  of  stratified  rocks,  give  evidence  of  having  been  ejected  to  the  surface  in  a  liquid  state — 
it  would  appear  that  their  emission  has  been  a  slow  and  mainly  a  quiet  process  of  long  duration,  hardly  attended  by  those  con- 
vulsions and  paroxysms  which  form  the  prominent  features  of  volcanic  action,  and  should  have  been  no  less  characteristic  of 
massive  eruptions  if  they  hud  been  due  in  any  large  measure  to  the  expansive  force  of  vapor.  The  process  of  the  emission 
of  the  rocky  matter  has,  it  is  true,  been  evidently  intermittent  in  most  cases,  as  may  be  inferred  from  the  occurrence  of  vast 
accumulations  of  breccia,  and  it  appears  that  an  extensive  solfataric  action  has  frequently  taken  place  through  neighboring 
fissures  ;  but  the  manner  in  which  the  matter  was  protruded  through  the  mnin  fissures  and  deposited  on  the  surrounding  parts 
of  the  surface,  had  evidently  no  similarity  to  the  mode  of  ejection  of  scoria,  ashes,  aud  lava  from  most  of  the  active  volca- 
noes. The  mode  of  action  described,  which  may  be  inferred  from  geological  observation,  is  perfectly  in  accordance  with  what 
we  should  expect  it  to  have  been  by  reasoning  a  priori  on  the  basis  of  our  previous  suppositions.  For,  if  G.  Bishof 's  cal- 
culation is  correct,  that  the  elastic  force  of  steam  is  8t  its  maximum  when  it  has  the  same  density  as  water,  which  it  would 
acquire  under  a  pressure  of  8,300  atmospheres,  the  loftiest  column  of  lava  (taking  its  mean  specific  gravity  to  be  3)  which 
should  be  supported  by  it,  would  be,  according  to  Jukes,  88,747  feet.  The  original  seat  of  those  eruptive  rocks  which  were 
protruded  without  being  accompanied  by  volcanic  action,  must  necessarily  have  been  at  a  much  greater  depth,  and  we  should 
therefore,  also  from  this  point  of  view,  be  led  to  suppose  that  the  expansive  force  of  steam  has  had  only  an  insignificant  part 
in  their  protrusion  from  greater  to  lesser  depth.  It  must,  of  course,  have  come  into  action  when  the  liquid  masses  arrived 
near  the  surface,  but  will  have  caused  hardly  more  than  an  ebullition,  even  in  viscid  masses,  on  account  of  the  extent  of  the 
openings.  The  formation  of  conglomerates  could  thereby  be  vastly  promoted,  but  their  final  deposition  and  consolidation 
must  have  been  quite  different  from  the  manner  in  which  similarly  subdivided  matter  would  be  deposited  around  a  volcanic 
orifice. 

(94) 


OP   VOLCANIC    ROCKS.  57 

demonstrate  in  another  chapter  that  the  changes  of  level  which  have  been  connected 
with  the  ejection  of  rocks,  appear  to  confirm  these  suppositions.  Suffice  it  here  to 
draw  the  necessary  consequence,  that  the  increasing  tension  must  finally  have  had  the 
effect  of  rupturing  the  newly  consolidated  masses.  If  in  the  intermediate  time  the 
masses  filling  the  fissures  of  the  first  epoch  had,  at  least  partially,  been  consolidated, 
then  a  new  system  of  fractures  would  be  opened,  within  the  limits  of,  though  not  coin- 
ciding with,  the  first.  These  fractures  of  the  second  epoch  would  descend  to  greater 
depth  than  those  of  the  first,  and  in  allowing  the  access  of  water  to  masses  situated  in 
lower  regions  and  being  of  a  more  basic  composition,  would  open  for  these  the  way  to 
the  surface.  By  the  repeated  occurrence  of  this  or  similar  processes,  the  theater  of 
action  from  which  the  rocky  masses  were  conducted  to  the  surface,  might  descend,  by 
steps,  into  considerable  depth  within  a  comparatively  short  period,  and  thus  there 
could  be  produced  a  great  diversity  among  the  rocks  emitted  through  -one  system  of 
fractures,  though  this  diversity  would  be  regulated  by  definite  relations  in  regard  to 
the  nature  and  succession  of  the  rocks  ejected.  The  process  would  come  to  an  end 
when  the  solidification  of  matter  and  the  formation  of  fractures  had  descended  to  those 
masses,  the  state  of  aggregation  of  which  was  such  as  no  longer  to  allow  them  to  crys- 
tallize when  the  pressure  was  diminished,  and  in  this  way  any  further  increase  of 
volume  would  be  prevented.  The  matter  filling  the  fractures  would  now  solidify,  and 
the  communication  of  the  interior  with  the  surface  be  cut  off,  with  the  exception  of 
the  volcanic  channels.  The  resistance  offered  by  the  crust  of  the  globe  would  hence 
be  greater  than  it  had  been  before,  and  there  would  follow  another  era  of  repose, 
longer  than  that  which  had  preceded  the  era  of  eruptive  activity. 

The  application  which  may  be  made  of  these  processes  suggested  by  theory,  to 
the  explanation  of  the  actual  correlations  of  eruptive  rocks  in  regard  to  their  age, 
chemical  composition  and  geographical  distribution,  is  obvious.  We  may,  indeed, 
venture  to  deduce  a  priori  the  history  of  eruptive  action,  in  its  main  features,  from  the 
hypothesis  of  Sartorius,  and  the  assumption  that  silicates  will  increase  in  volume  by 
perfect  crystallization.  We  should  have  to  conclude  that  in  a  remote  period,  when 
the  crystallized  crust  and  the  sedimentary  shell  of  the  globe  were  inferior  in  aggregate 
thickness,  fractures  and  eruptions  of  rocky  matter  would  have  been  of  frequent  occur- 
rence, and  that  highly  silicious  compounds  should  have  prevailed  among  the  ejected 
masses.  Little,  if  anything  of  them,  is  probably  visible  at  the  present  surface,  as  the 
rocks  of  the  Azoic  and  Paleozoic  formations  are  probably  the  monuments  of  an  already 
far  advanced  stage  of  the  development  of  our  planet.  All  the  distinguishing  features 
of  the  eruptive  rocks  of  these  periods  (including  of  the  Palaeozoic  age  only  the  Silurian 
and  the  first  part  of  the  Devonian),  such  as  the  great  number  and  individualization  of 
the  granitic  districts,  the  independence  of  each  of  them  in  the  subtler  differences 
regarding  the  commencement  and  further  development  of  the  eruptive  activity  as  well 
as  the  peculiar  nature  of  the  rocks  ejected,  the  great  preponderance  of  highly  silicious 
compounds,  the  common  association  with  them  of  small  quantities  of  basic  rocks,  and 
the  slight  increase  of  the  proportion  of  the  latter  in  the  Devonian  period — all  these 
phenomena  are  easily  understood,  without  further  explanation,  in  the  light  of  our 

hypothesis. 

(95) 


58  RICIITIIOFEN NATURAL   SYSTEM 

Let  us  now  direct  our  attention  at  once  to  the  volcanic  era.  The  conditions 
of  the  globe  must  have  been  very  different  in  the  Tertiary  from  what  they  had 
been  in  the  Palaeozoic  period.  A  longer  time  of  comparative  repose  had  in  most  parts 
of  the  globe  preceded  the  inauguration  of  the  violent  manifestations  of  vulcanism  in 
the  Tertiary  period  than  had  ever  before  elapsed  between  any  two  eras  of  eruptive 
activity.  The  globe  had  cooled  down.  Volumes  of  sedimentary  matter  had  accumu- 
lated, and  added  externally  to  the  thickness  of  its  crust,  while  it  had  increased  in  a 
vastly  greater  measure  by  the  crystallization  of  liquid  matter  below.  Those  silicious 
compounds  especially,  of  low  specific  gravity,  which  had  formerly  yielded  the  material 
of  the  vast  accumulations  of  quartziferous  eruptive  rocks,  would  have  been  consoli- 
dated, and  the  limit  as  it  were  between  the  solid  and  the  viscous  state  of  aggregation 
receded  into  regions  where  the  matter  would  be  of  a  less  silicious  composition  and  of 
greater  specific  gravity.  The  similarity  in  distant  countries  of  the  rocks  first  ejected 
(propylite  and  andesite)  goes  to  show  that  the  recession  of  that  limit  into  greater 
depth  must  have  proceeded  in  a  nearly  equal  ratio  in  all  those  regions  where  volcanic 
rocks  are  distributed.  When  the  tension  below  had  increased  sufficiently  to  overcome 
the  resistance,  it  would  now  no  longer  manifest  itself  in  the  formation  of  small  and 
differentiated  systems  of  ruptures.  In  the  direct  ratio  of  the  increase  of  the  resist- 
ance the  fractures  would  have  to  be  of  greater  extent,  and  those  elongated  belts  of 
them  would  be  formed  which  even  now  are  partially  distinguished  as  the  belts  of  vol- 
canic activity.  The  first  rocks  ejected  would  necessarily  be  of  a  more  basic  composition 
than  the  predominant  rocks  of  the  granitic  era,  while  the  repetition,  at  a  later 
epoch,  of  the  process  of  fracturing  would  give  rise  to  the  ejection  of  rocks  in 
which  silica  would  be  contained  in  a  still  lower  proportion.  The  greater  portion 
indeed  of  the  ejected  rocks  consisted  of  propylite  and  andesite,  in  the  first,  and  of 
basalt  in  the  second  half  of  the  volcanic  era.  A  notable  but  only  apparent  anomaly 
in  the  regular  order  of  succession  has  been  the  emission  of  trachyte  and  rhyolite 
between  the  andesitic  and  basaltic  epochs.  But  if  it  is  considered  that  these  rocks  were 
ejected  partly  from  the  same  fractures  through  which  andesite  had  ascended,  and 
partly  from  others  in  their  immediate  vicinity,  while  the  distribution  of  basalt  has 
been  independent,  to  a  certain  extent,  of  all  foregoing  eruptions,  it  is  evident  that 
the  occurrence  of  trachyte  and  rhyolite  is  closely  dependent  on  that  of  andesite,  and 
bears  only  a  very  remote  relation  to  basalt.  It  appears  that  after  the  ejection  of  the 
chief  bulk  of  andesite.  when  other  processes  ending  in  the  opening  of  fractures  into 
the  basaltic  region  were  being  slowly  prepared  in  depth,  the  seat  of  eruptive  activity 
ascended  gradually  toregions  at  less  distance  from  the  surface.  There  is,  within  the 
limits  of  conjecture  based  on  physical  laws,  no  lack  of  processes  which  could  cooperate 
to  that  effect.  The  consolidation  of  the  ejected  masses  within  the  fissures  would  prob- 
ably proceed  simultaneously,  by  loss  of  heat,  from  the  surface  downwards  and,  by 
pressure,  from  below  upwards.  The  opening  of  new  branches  from  the  main  frac- 
tures, the  remelting  (by  the  aid  of  the  heat  of  the  molten  mass  within  the  latter,  and 
of  water  finding  access  to  it)  of  solidified  matter  adjoining  the  fracture,  the  emission 
of  that  remelted  matter  through  those  branches  :  all  these  are  secondary  processes 

depending  on  the  first  almost  necessarily.     The  supposition  that  to  these  is  due  the 
(9fi) 


OF   VOLCANIC   ROCKS.  59 

order  of  time  in  which  trachyte  and  rhyolite  have  been  ejected  to  the  surface,  is 
corroborated  by  the  fact  that  these  rocks  occupy  generally  a  subordinate  position  in 
regard  to  quantity,  and  have  had,  to  a  great  extent,  their  origin  in  volcanic  action. 
When  treating  about  the  latter,  we  will  come  back  upon  this  subject.  There  remain 
some  peculiar  features  of  volcanic  rocks  which  cannot  be  satisfactorily  explained  at 
the  present  time.  We  mention,  among  them,  the  fact  that  the  three  modes  of  texture 
of  rhyolitic  rocks  are  often  severally  limited  to  certain  localities  ;  the  mode  of  forma- 
tion of  the  laminated  structure  of  rhyolitic  and  trachytic  rocks  ;  the  occurrence  of  the 
compounds  of  hornblende  and  oligoclase  in  that  threefold  form  to  which  we  have 
repeatedly  referred ;  the  fact  that  basalt  has  been  followed  only  to  a  very  limited 
extent  by  rocks  bearing  to  it  a  similar  relation,  as  trachyte  and  rhyolite  do  to  andesite. 

As  regards  the  long  lapse  of  time  intermediate  between  the  Devonian  and  the 
Tertiary  periods,  the  mode  of  occurrence  of  eruptive  rocks  in  the  same  shows  in 
nearly  every  respect  a  gradual  transition  from  that  which  was  peculiar  to  the  granitic  to 
that  which  we  just  described  as  being  characteristic  of  the  volcanic  era.  This  interme- 
diate period  may  be  designated  as  the  porphyritic  era,  though  this  name  appears  to 
apply  more  properly  to  its  first  part  only.  Quartziferous  rocks  were  not  so  predom- 
inant in  it  as  in  the  granitic  era,  porphyrite  and  melaphyr  having  nearly  equaled 
quartzose  porphyry,  in  point  of  quantity.  Augitic  porphyry  was  ejected  in  a 
much  larger  proportion  to  the  aggregate  bulk  of  the  porphyritic,  than  diabase  to 
that  of  the  granitic  rocks.  Where  it  occurs,  it  was  the  last  in  the  order  of  rocks 
erupted,  while  quartzose  porphyry  was  generally  the  first  among  them,  though  this 
place  is  sometimes  occupied  by  porphyrite.  In  reference  to  the  general  features  of 
their  geographical  distribution,  porphyritic  rocks  occupy  no  less  distinctly  an  inter- 
mediate position,  as  may  be  seen  by  what  we  have  said  on  this  topic  on  another  page. 

There  are  exceptions  to  the  order  of  g'eneral  development  as  here  specified. 
They  regard  chiefly  the  texture,  and  are  almost  exclusively  to  be  found  among  the 
rocks  of  the  porphyritic  era,  though  the  recurrence,  in  propylite,  of  the  properties  of 
ancient  diorite,  is  a  phenomenon  of  a  no  less  exceptional  nature.  Leaving  this  rock 
(propylite),  or  rather  only  some  of  its  varieties,  out  of  consideration,  the  volcanic  rocks 
have  their  peculiar  characters,  by  which  even  the  most  basic  rocks  are  to  be  recog- 
nized when  seen  in  large  accumulation.  In  the  rocks  of  the  granitic  era,  if  we 
consider  its  end  to  be  within  the  Devonian  period,  the  characteristic  features  of  por- 
phyritic and  volcanic  rocks  are  probably  never  to  be  observed.  Among  the  excep- 
tional occurrences  within  the  porphyritic  era,  may  first  be  noticed  the  fact  that  perfect 
granitic  texture  is  still  occasionally  encountered,  as,  for  instance,  near  Predazzo  and 
on  the  Monzoni  in  southern  Tyrol,  where  some  subordinate  masses  of  rock  resembling 
granite  and  syenite  have  been  ejected  in  the  Triassic  age,  together  with  the  well-known 
porphyritic  rocks  of  that  region.  There  are  similar  instances  known  from  other  places 
on  the  European  continent,  but  they  are  scattered,  and  the  respective  rocks  always 
quite  limited  in  extent.  The  grandest  exceptional  instance  that  is  known  up  to  this 
time,  is  the  recurrence  in  the  Jurassic  period  of  perfect  granitic  texture  in  the  erup- 
tive rocks  of  the  Sierra  Nevada.  But  as  regards  their  mineral  composition,  these  rocks 
belong  to  the  family  of  syenitic  granite,  containing  hornblende  as  a  very  characteris- 

(97) 


60  RICHTHOFEN NATURAL   SYSTEM 

tic  ingredient,  while  silica  enters  into  their  average  composition  probably  in  a 
much  lower  proportion  than  it  is  contained  in  ordinary  granite.  The  varieties  known 
as  granitite  are  of  rare  occurrence.  In  another  respect,  namely,  the  mode  of  geograph- 
ical distribution,  the  Jurassic  granite  of  the  Andes  marks  an  advanced  stage,  if  com- 
pared with  the  Permian  and  Triassic  porphyries  of  Europe,  inasmuch  as  the  area  of 
its  distribution  excels  the  porphyritic  regions  of  the  latter  continent  in  regard  to  their 
extent  and  the  unity  of  their  inner  relations,  while  it  does  not  come  up,  in  these 
respects,  to  the  properties  of  the  great  volcanic  belts. 

2.     Origin  of  Volcanic  Action. 

I  have  tried  to  demonstrate  in  another  chapter,  that  volcanic  activity  is  inti- 
mately connected  with  massive  eruptions  in  a  three-fold  way,  namely,  in  respect  to 
the  epoch  of  its  commencement,  the  mode  of  its  distribution,  and  the  nature  of  the 
rocks  ejected.  Basaltic  volcanoes  occur  in  ranges  built  up  by  the  massive  eruptions  of 
basalt,  or  in  their  vicinity,  in  such  connection  as  to  make  obvious  the  nearly  contem- 
poraneous origin  of  both.  Andesitic  volcanoes  are  found  in  the  neighborhood  of  those 
masses  of  andesite  which  had  been  ejected  without  being  attended  by  volcanic  action  ; 
while  volcanoes  which  have  emitted  rhyolitic  or  trachytic  lava  are  so  situated  as  to 
justify  the  inference  of  a  close  connection  between  the  first  opening  of  their  vents 
and  the  origin  of  neighboring  accumulations,  either  of  rocks  of  similar  character  or  of 
andesite,  but  which  are  due  to  massive  eruptions.21 

No  distinct  line  of  demarcation  can  be  drawn  between  the  two  modes  of  mani- 

21  This  affinity  in  regard  to  petrographical  character  and  geographical  distribution  is  probably  the  cause  why  both 
agencies  have  ordinarily  been  confounded.  It  has  been  a  current  notion,  strongly  advocated  at  times,  that  all  volcanic 
rocks  have  come  to  the  surface  in  a  similar  way  to  that  in  which  lava  is  being  ejected  from  volcanic  vents,  and  that  the 
cause  of  the  ejection  has  been  the  same  in  all  cases.  In  regard  to  the  fact  that  extensive  mountain  ranges  are  completely 
built  up  of  volcanic  rocks,  it,  was  argued  that  it  is  by  no  means  necessary,  and  in  fact  erroneous,  to  suppose  them  to  have 
originated  in  events  surpassing  in  magnitude  those  of  the  present  day,  as  the  length  of  geological  time  would  explain  how 
they  could  be  built  up  by  the  gradual  accumulation  of  innumerable  currents  of  lava.  Where  traces  of  former  volcanic 
vents  could  not  be  found,  the  easy  destructibility  of  the  matter  of  which  the  sides  of  craters  are  usually  composed,  afforded 
a  convenient  and  apparently  just  argument  for  explaining  their  absence  by  denudation.  The  same  doctrine  was  applied  for 
explaining  the  mode  of  formation  of  ancient  eruptive  rocks  where  we  see  mountain  masses  made  up  of  them,  while  others 
have  arrived  at  the  conclusion  that  they  were  the  "  roots  "  or  "  cores  "  of  volcanoes ;  granite  itself  is  by  them  considered 
to  have  had  formerly,  and  to  have  now  this  function,  even  in  the  case  of  those  volcanoes  the  lava  of  which  consists  of  basalt. 
Both  these  doctrines  have  been  adopted  the  more  readily,  as  they  appear  to  be  in  harmony  with  the  favorite  hypothesis,  that 
at  no  time  have  any  changes  on  the  surface  of  the  globe  been  more  violent  than  those  going  on  at  the  present  day,  nor 
different  from  them  in  mode.  However  ably  this  theory,  which  contributed  so  much  to  the  advancement  of  science  by 
checking  the  phantasmagorias  of  former  time,  has  been  advocated,  an  unbiased  comparison  of  the  grand  manifestations 
recorded  in  the  geological  structure  of  an  andesitic  mountain  range,  with  the  mode  and  degree  of  activity  of  present  vol. 
canoes,  must  lead  to  different  conclusions.  In  the  endeavor  to  sustain  the  a  priori  assumption  of  the  equality  of  force  in  all 
ages,  too  little  stress  has  been  laid  on  the  circumstance,  that,  however  nearly  equal  the  aggregate  amount  of  force  acting  on 
the  globe  may  have  been,  its  modes  must  have  undergone  a  change,  chiefly  by  the  partial  conversion  of  the  heat  of  the 
globe  into  other  forces.  At  the  same  time,  the  degree  of  the  intensity  of  the  manifestations  of  subterranean  forces  must 
have  varied  in  a  two-fold  way.  With  the  continuous  increase  of  resistance,  the  aggregate  motions  of  the  crust  must  have  de- 
creased, since  a  growing  amount  of  these  forces'was  required  for  overcoming  the  resistance.  On  the  other  hand,  the  manifest- 
ations of  the  same  forces  had  to  become  periodical  and  paroxysmal,  and  periods  of  violent  action  had  to  be  separated  by 
others  of  repose.  The  violent  activity  of  the  Tertiary  period  has  passed,  and  our  present  volcanoes  appear  to  mark  the  transi- 
tion into  another  period  of  repose.  I  will  try  to  demonstrate  that  the  causes  of  their  action  are  utterly  inadequate  for  ex- 
plaining the  grander  phenomena  and  correlations  of  massive  eruptions. 
(98) 


OF   VOLCANIC   ROCKS.  61 

festation  of  subterranean  energy,  as  some  of  the  monuments  of  the  grand  volcanic 
action  of  the  past  indicate  the  former  existence,  at  certain  places,  of  a  stage  intermedi- 
ate between  both.  But  in  those  numerous  instances  where  the  nature  of  either  of 
them  can  be  distinctly  recognized,  some  conspicuous  differences  may  be  noted  between 
them.  Volcanoes  are  provided  with  a  channel  connecting  the  seat  of  volcanic  action 
with  the  surface.  The  matter  which  they  eject  consists  either  of  stratified  layers  of 
ashes  and  scoria,  or  of  currents  of  lava  in  the  shape  of  flat  sheets  superimposed  over 
one  another,  or  of  alternating  layers  of  both  kinds  of  material,  the  latter  structure 
being  most  frequent.  A  low  dip,  verging  on  horizontality,  of  all  planes  dividing  sub- 
stances of  dissimilar  nature,  or  marking  successive  deposition,  and  homogeneity  of 
material  parallel  to  those  planes,  together  with  a  structure  of  the  mountain  masses 
distinguished  by  the  radiation  of  all  mineral  matter  from  one  common  center,  may  be 
considered  as  those  features  by  which  even  extinct  volcanoes,  or  fragments  of  them, 
may  unmistakably  be  recognized.  The  center  may  shift,  or  have  shifted,  within  nar- 
row limits,  or  a  series  of  centers  may  follow  each  other  in  close  succession,  but  this 
will  obliterate  the  true  character  only  in  a  slight  degree.  Similar  rocks,  when  they 
came  to  the  surface  by  massive  eruptions,  do  not  present  these  distinguishing  features. 
They  usually  compose  ranges  of  small  width  in  proportion  to  their  length,  and  in  the 
place  of  one  or  more  distinct  centers  an  elongated  axis  may  be  detected,  from  which 
the  structural  and  morphological  features  originate.  As  regards  the  interior  structure, 
there  may  be  observed  a  certain  massive  character  of  the  rocks,  which  is  partly  pro- 
duced by  the  prevalence  of  their  compact  varieties,  and  partly  by  the  circumstance 
that  homogeneity  of  lithological  character  may  be  traced  to  a  great  distance  in  a  verti- 
cal direction.  In  sections,  masses  are  frequently  found  thousands  of  feet  in  height, 
which  do  not  vary  perceptibly  in  character,  and  show  no  horizontal  structure.  If  the 
slopes  of  the  ranges  are  examined,  the  rocks  will  be  found  preserving  a  homogeneous 
character  chiefly  in  a  direction  parallel,  to  the  axis,  while  it  is  less  persistent  at  right 
angles  to  the  latter.  The  planes  dividing  dissimilar  rocks  are  inclined  at  all  angles, 
and  have  very  frequently  a  steep  or  nearly  vertical  position.  -Breccias  even,  which 
sometimes  occur  in  very  large  masses,  are  bounded  in  this  way  towards  the  adjoining 
compact  varieties  ;  they  are  of  irregular  shape,  and  do  not  often  occur  in  stratified 
layers  or  in  elongated  currents,  as  is  the  case  when  they  are  produced  by  volcanic 
action.  The  ranges  made  up  by  massive  eruptions  show  no  signs  of  craters.  Yet 
they  are  frequently  the  foundation  of  volcanoes.  Oftener  still  do  volcanoes  occur  on 
the  lower  portions  of  their  slopes,  or  they  may  form  a  series  parallel  to  the  axis  of 
the  main  range,  and  even  greatly  exceed  it  in  elevation. 

Notwithstanding  these  points  of  difference,  there  are  not  only  stages  of  transi- 
tion between  the  distinct  geological  features  resulting  from  either  mode  of  action,  but 
a  similarity  in  character  may  be  produced  under  certain  circumstances,  which  makes  it 
difficult  to  decide  what  was  the  mode  of  origin  of  an  accumulation  of  volcanic  rocks. 
In  the  first  place,  the  matter  extruded  through  fissures  may  have  been  so  liquid  as  to 
expand  at  once  in  thin  sheets.  This  is  very  frequently  the  case  with  basalt,  in  the 
great  accumulations  of  which,  whether  they  be  due  to  massive  eruptions  or  to  volcanic 
activity,  the  prominent  differences  wrought  by  both  modes  of  action  in  the  external 
M  (99) 


62  RICUTHOFEN — NATURAL   SYSTEM 

features  are  generally  least  conspicuous.  This  basic  compound  is  not  only  more  fusi- 
ble than  the  more  silicious  rocks ;  but,  it  appears  that  the  admixture  of  superheated 
water  will  increase  its  fluidity  more  than  that  of  the  other  compounds,  while  those 
unknown  influences  which  cause  the  mass  to  solidify  in  the  particular  form  of  basalt 
must  be  still  more  potent  in  increasing  its  fluidity,  since  lavas  consisting  of  dolerite, 
leucitophyre,  and  trachydolerite,  which  are  quite  or  nearly  identical  with  the  former 
in  chemical  composition,  are  never  so  liquid  as  those  of  basalt,  and  are  not  unfrequently 
quite  viscous.  These  rocks,  together  with  all  those  of  a  more  silicious  composition,  ex- 
hibit more  distinctly  the  differences  in  origin,  and  this  is  probably  due,  in  a  great  meas- 
ure, to  the  peculiarity  just  mentioned.  The  great  fluidity  of  basalt,  which  is  also  made 
evident  by  its  frequent  occurrence  in  very  thin  and  yet  very  extended  dykes,  causes 
it,  even  when  confined  in  the  narrow  space  of  a  volcanic  vent,  to  let  the  vapors  of 
water  escape,  in  quiet  ebullition,  in  its  orifice,  as  Dana  has  so  beautifully  illustrated  in 
his  description  of  Kilauea,  while,  at  intervals,  it  will  break  out  and  cover  the  sur- 
rounding country  with  flat  sheets  of  lava.  The  action  connected  with  the  ejection  of 
the  same  rock  from  larger  fissures,  in  former  time,  appears  to  have  been  similar  to 
this.  Yet,  the  numerous  instances  of  the  occurrence  of  basaltic  cinder  cones,  and,  on 
the  other  hand,  of  large  accumulations  of  solid  basalt  with  no  perceptible  horizontal 
structure,  go  to  show  that  also  in  the  case  of  this  rock  the  modes  of  occurrence  may 
be  different  when  they  are  the  result  of  different  modes  of  ejection.  Other  instances 
of  a  similarity  of  the  manner  in  which  the  matter  has  been  deposited,  when  due  to 
either  mode  of  ejection,  are  frequent  on  the  flanks  and  at  the  ends  of  the  andesitic 
ranges  of  Hungary,  where  currents  of  andesite  as  well  as  of  rhyolite  have  been  emitted 
through  fissures  in  andesite,  at  little  elevation  above  the  foot  of  those  ranges.  They 
appear  to  be  due  to  processes  intermediate  in  kind  between  both  modes  of  ejection. 
We  may,  finally,  mention  those  cases  where  massive  eruptions  were  sub -aqueous,  and 
layers  of  fine-grained  tufa  formed,  alternating  with  coarser  conglomerates,  between 
which  may  be  intercalated  solid  layers  of  the  same  kind  of  rock  of  the  fragments  of 
which  those  are  composed.  The  similarity  of  this  kind  of  depositions  with  the  sedi- 
ments of  submarine  volcanoes  is  often  very  great. 

The  principal  point  of  difference  between  massive  eruptions  and  volcanic  action 
appears  to  be  the  depth  of  their  source  under  the  surface,  and  all  the  minor  differences 
are  probably  dependent  upon  that.  The  region  from  which  the  former  have  derived 
their  material,  is,  as  we  tried  to  prove,  at  a  great  depth  beneath  the  deepest  sedimen- 
tary rocks.  The  seat  of  volcanic  action  appears  to  vary  within  wide  limits  in  regard  to 
its  distance  from  the  surface,  but  to  be,  on  an  average,  at  much  less  depth  than  that  of 
the  massive  eruptions  ;  though  there  are  circumstances  which  render  it.  probable  that 
it  is  in  all  cases  beneath  the  shell  composed  of  sediments.  Evidence  has  been  gathered 
by  Prevost,  Dana,  Scrope,  Hopkins,  and  others,  in  favor  of  the  assumption  that  vol- 
canoes are  not  connected  with  the  molten  interior  of  the  globe,  and  are  therefore  not 
to  be  considered  as  safety-valves.  The  comparatively  little  distance  of  the  seat  of  vol- 
canoes beneath  the  surface  is  rendered  particularly  evident  by  the  small  area  of  the 
earthquakes  attending  their  activity,  when  compared  with  the  wide  extent  of  others 
which  must  be  dependent  on  some  deep-seated  action,  but  have  no  recognizable  con- 
(100) 


OF    VOLCANIC   ROCKS.  63 

nection  with  any  particular  volcanic  vent.  It  is  no  less  obvious  from  the  fact  that  two 
neighboring  volcanoes  may  not  only  eject,  contemporaneously,  different  kinds  of  lava, 
but  also  be,  to  some  extent,  independent  of  each  other  in  their  manifestations. 

We  may  base  further  conclusions  in  regard  to  the  origin  of  volcanoes  upon  the 
following  premises,  which  we  repeat  from  the  foregoing  pages :  First,  several  facts 
appear  to  indicate  that  the  source  of  volcanic  action  is  at  a  comparatively  limited 
depth  ;  second,  all  volcanoes,  whether  active  or  extinct,  are  intimately  connected  with 
massive  eruptions  ;  third,  this  connection  is  of  such  a  character  as  to  establish,  in  the 
majority  of  cases,  the  close  similarity  and  chemical  identity  of  the  mineral  matter 
ejected  by  the  volcano  in  its  first  epoch  of  activity  with  that  of  neighboring  hills  or 
of  its  own  foundation,  which  had  been  accumulated  by  massive  eruptions  ;  while  in 
other  cases  these  neighboring  hills  or  the  foundation  of  the  volcano  are  composed  of 
volcanic  rocks  not  identical  with  the  lava,  and  then  the  latter  will  belong  lithologically  to 
a  kind  of  rock  which,  according  to  the  order  of  succession  of  massive  eruptions,  would  be 
of  a  more  recent  origin  than  the  former ;  that  is,  trachytic  or  rhyolitic  volcanoes  were 
frequently  opened  at  those  places  where  only  andesite  had  been  accumulated  before, 
and  basalt  where  either  andesite  or  trachyte  had  preceded  ;  but  the  reversed  order 
appears  not  to  occur,  no  basaltic  volcano  having  been  succeeded,  in  its  own  neighbor- 
hood, by  massive  eruptions  of  trachyte  or  andesite  ;  fourth,  many  of  those  volcanoes 
which  have  been  active  through  a  long  period,  have  undergone  a  periodical  change 
in  regard  to  the  character  of  the  mineral  matter  ejected  by  them,  and  this  change  is 
in  general  (though  with  exceptions)  conformable  to  the  order  of  succession  observed 
in  regard  to  massive  eruptions. 

From  these  facts  may  be  inferred  the  complete  dependency  of  volcanoes  upon 
massive  eruptions.  The  latter,  as  we  attempted  to  show,  were  due,  firstly,  to  the  open- 
ing of  systems  of  fissures  which  extended  throughout  the  solid  crust ;  and  secondly,  to  a 
quiet  outflow,  which  was  caused  by  the  expansion  attending  the  change  of  aggregation 
of  solid  or  highly  viscous  matter  around  the  lowest  part  of  the  fissures  into  that  of 
aqueous  fusion.  The  relations  of  volcanic  activity  to  massive  eruptions  are  indicative 
of  a  process  by  which  the  elongated  and  extensive  vents  of  the  latter  were  gradually 
differentiated  into  isolated  and  narrow  channels  feeding  isolated  orifices  on  the  surface. 
It  has  been  observed  that  cinder  eruptions  mark  generally  the  last  stage  of  volcanic 
action.  We  may  go  a  step  further  back,  and  say  that  volcanic  action  is  the  last  stage 
of  massive  eruptions. 

In  order  to  arrive  at  a  conception  of  the  manner  in  which  the  change  from  one 
mode  of  action  to  the  other  could  be  effected,  let  us  suppose  that  a  main  fissure  was 
filled  with  matter  from  below,  and  mountains  of  volcanic  rocks  accumulated  above  it 
by  the  long  continued  overflow.  Solidification  would  at  once  set  in,  and  proceed 
downward  whenever  a  cessation  of  the  extrusive  action  occurred,  independent  of 
the  question  whether  it  would  not  simultaneously  proceed  upwards  from  the  depth. 
Its  progress  would  not  be  equal  in  all  parts  of  the  fissure,  since  this  must  be 
wider  at  some  places,  and  more  contracted  at  others.  In  this  condition  we  should 
have  one  of  the  causes  for  the  isolation  of  centers  of  action,  for  the  length  of  time 
during  which  different  portions  of  the  matter  would  remain  in  a  fluid  condition,  must,  of 

(101) 


64  RICHTHOFEN NATURAL   SYSTEM 

course,  depend  in  a  great  measure  upon  the  width  they  would  severally  occupy  within 
the  channels,  and  those  filling  its  wider  parts  must  for  a  longer  time  remain  susceptible 
of  renewed  expansion  by  accidental  circumstances.  Another  cause  which  would  have 
the  same  general  effect,  is  the  localization  of  the  ingress  of  water.  Some  of  its  chan- 
nels would,  to  all  probability,  become  obstructed,  and  the  rate  at  which  new  ones 
would  be  opened  in  their  place  would  probably  diminish  in  nearly  equal  ratio  with  the 
total  amount  of  the  manifestations  of  energy  connected  with  the  phenomenon  of  ejection 
in  its  different  stages.  As  'it  appears  that  the  supply  of  water,  which  may  be  either 
constant  or  intermittent,  is,  next  to  an  elevated  temperature,  the  chief  condition  for 
entertaining  volcanic  action,  it  may  be  inferred  that  the  cause  mentioned  would  contrib- 
ute greatly  towards  the  isolation  of  certain  portions  within  the  main  fissure,  by  helping 
to  keep  the  matter  within  them  in  a  liquid  state.  "If  this  second  cause  was  coincident 
with  the  first,  by  the  restriction  of  the  ingress  of  water  to  a  place  where  the  fissure 
expanded,  then  both  circumstances  would  combine  to  prolong  the  state  of  liquidity  at 
that  point.  The  connection  between  the  bottom  and  the  surface  may  have  been 
kept  open  in  a  certain  part  of  the  mouth  of  the  fissure,  while  solidification  was  proceed- 
ing over  the  rest  of  it.  An  isolated  vent  would  then  gradually  be  formed,  and  nar- 
rowed down  to  the  size  of  a  volcanic  orifice.  Obstructions  of  the  outflow,  by  periodical 
consolidation,  would  become  more  frequent,  and  thus  would  proceed  a  slow  change  of 
the  mode  of  action  of  massive  eruptions  to  that  which  is  peculiar  to  volcanoes. 

This  is  probably  the  simplest  manner  in  which  volcanoes  can  originate.  It  will 
apply  particularly  to  a  number  of  those  which  have  undergone  no  change  in  regard  to 
the  character  of  their  lava,  and  the  lofty  cones  of  which  rise  over  mountain  ranges 
consisting  of  the  same  material  with  their  own  lava  and  cinders,  though  owing  their 
origin  to  massive  eruptions.  We  must  now  consider  a  third  cause  which  would  aid  in 
promoting  volcanic  action,  and  probably  come  very  often  into  play.  It  is  indicated  by 
the  frequent  occurrence  of  series  of  volcanoes  extending  in  lines  parallel  to  the  axis  of 
the  main  outbursts.  Their  only  possible  cause  is  the  formation  of  fractures,  parallel  to 
the  main  fissure,  and  branching  off  from  it.  In  order,  however,  that  these  could  be 
formed,  solidification  must  have  proceeded  downward  in  the  main  fissure,  without  any 
communication  with  the  liquid  portion  in  depth  having  been  kept  open.  This  process 
would  necessarily  imply  a  temporary  cessation  of  the  process  of  extrusion.  That  this 
could  take  place  may  be  the  more  readily  understood,  if  it  is  taken  into  consideration, 
that  the  liquid  masses  filling  the  fissure  in  its  whole  extent  must  contract  considerably 
by  constant  loss  of  heat,  and  that  any  additional  expansion  produced  by  the  promotion 
of  aqueous  fusion  at  certain  places  had  first  to  equalize  this  loss  of  volume,  before 
it  could  manifest  itself  in  a  rise  of  the  whole  mass.  This  twofold  action,  which  is 
probably  one  of  the  main  causes  of  the  intermittent  character  of  volcanic  ac- 
tivity, must  produce  an  alternating  motion  of  matter  within  the  fissure,  and 
there  would  be  given  ample  opportunity,  during  a  period  of  its  subsidence,  for 
the  consolidation  of  the  upper  portion  to  a  great  distance  down  from  the  surface. 

If  a  period  then  followed  in  which,  by  dislocations  of  some  kind,  a  change  took 
place  in  the  conditions  subterranean,  and  expansion  began  again  to  prevail  within  the 
fissure,  the  new  supply  of  force  would  manifest  itself  at  the  upper  limit  of  the  liquid 
(102) 


OF    VOLCANIC    ROCKS.  65 

portion,  where  the  least  resistance  was  offered,  and  new  fractures  would  be  opened 
branching  off  at  that  depth  from  the  main  fissure.  The  liquid  matter  would  ascend 
through  these  secondary  fractures,  and,  if  these  were  of  sufficient  dimensions,  give  rise 
first  to  the  formation  of  parallel  ranges  of  volcanic  rocks  by  massive  eruptions,  and 
then  only  to  a  gradual  isolation  of  channels  of  volcanic  action,  as  in  the  case  first  ex- 
plained ;  or,  if  the  fracture  consisted  of  a  series  of  smaller  ruptures,  cause  at  once  the 
formation  of  a  series  of  volcanoes.  The  activity  in  these  secondary  fissures  could  con- 
tinue long  after  any  manifestations  had  ceased  over  the  main  fissure,  and  even  after  the 
consolidation  of  the  matter  contained  in  this,  with  the  exception  of  the  volcanic  hearths, 
had  proceeded  into  far  greater  depth.  The  formation  of  secondary  fractures,  branch- 
ing off  from  the  main  fissure,  might  be  repeated  at  different  depths,  and  fissures  of  a 
third  order  be  formed,  branching  off  from  those  of  the  second.  Subterranean  reser- 
voirs of  liquid  matter,  which  may  either  be  isolated  or  connected,  would  thus  be  formed 
at  different  depths,  and  be  arranged  after  a  similar  plan  below  ground,  as  we  notice 
among  the  active  and  extinct  volcanic  orifices  above  ground.  The  hypothesis  of 
the  existence  of  such  subterranean  seas  of  melted  matter,  as  they  have  been  called,  has 
also,  though  in  a  very  different  meaning,  been  arrived  at  by  the  adherents  of  the  theory 
of  a  metamorphic  origin  of  volcanic  rocks.  This  coincidence  increases  the  degree  of  its 
probability.  But  unless  the  nature  and  distribution  of  those  reservoirs  is  made  de- 
pendent on  grander  phenomena  having  connection  with  the  interior  of  the  globe,  they 
will  not  be  capable  of  explaining  the  harmony  prevailing  either  in  one  volcanic  region 
or  between  all  these  regions.  We  have  been  led,  by  arguing  on  our  suppositions,  to 
the  same  conclusion  at  which  we  arrived  before  by  induction  from  observed  facts,  namely, 
that  the  seat  of  volcanic  action  must  be  at  a  comparatively  limited  depth.  Yet  it  ap- 
pears that  this  depth  is  in  all  cases  below  the  shell  of  sedimentary  rocks.  Among  the 
reasons  supporting  this  assumption,  we  mention  only  one.  This  relates  to  the  chemical 
composition  of  lava.  The  volcanic  is  typically  the  era  of  andesitic  and  basaltic  com- 
pounds. The  occurrence  of  trachyte  and  rhyolite  among  the  ejected  rocks  goes  to 
show,  that  the  seat  of  action  receded  at  certain  places  from  the  andesitic  regions  to 
those  of  the  compounds  corresponding  in  chemical  composition  to  the  two  kinds  of  rock 
named.  But  if  it  had  been  partially  above  those  regions  where  matter  is  still  in  its 
primaeval  position,  then  we  should  expect  that  there  would  be  volcanoes  the  lavas  of 
which,  being  derived  from  sedimentary  rocks,  would,  as  a  whole,  deviate  in  composition 
from  the  law  of  Bunsen.  No  such  volcanoes  are  known,  and  it  is,  therefore,  not  prob- 
able that  the  seat  of  any  of  those  the  lava  of  which  has  been  analyzed  is  within  the 
shell  of  sedimentary  rocks.  It  is  true  that  subordinate  deviations  from  the  composition 
as  required  by  theory  occur ;  but  it  has  been  found  sufficient  to  ascribe  them,  as  in  the 
case  of  Vesuvius,  to  a  mechanical  destruction  of  the  rocks  surrounding  the  channel  of 
ejection  by  the  ascending  lava. 

Active  volcanoes  themselves  furnish  an  illustration  in  evidence  of  their  own 
origin  as  here  advocated.  It  is  well  known  that  small  cones  are  frequently  met  with 
on  the  slopes  of  larger  volcanoes.  If  they  occur  in  larger  number,  as  on  Mount  Etna, 
they  are  usually  situated  in  lines  which  radiate  from  the  crater.  Each  of  them  is  built 
up  of  layers  of  scoria  and  ashes  sloping  away  from  the  center,  where  a  crater  is  im- 

(103) 


66  RICHTHOFEN  —  NATURAL    SYSTEM 

mersed,  and  such  cones  will  occasionally  emit  currents  of  lava,  and  be  in  fact  the  rep- 
etition on  a  small  scale  of  the  mother  volcano.  The  usual  and  probably  correct  ex- 
planation of  their  mode  of  occurrence  is  this  :  That,  through  crevices  or  by  passage 
through  porous  rocks,  water  gets  access  to  glowing  lava,  and,  by  its  action  on  the  same, 
causes  the  opening  of  a  fracture,  and,  in  immediate  succession,  the  repetition  of  the 
same  phenomena  which  the  mother-volcano  presents  when  active.  Just  as  these  par- 
asitic volcanoes  have  their  roots  in  the  glowing  lava,  volcanoes  in  general  must,  as  is 
demonstrated  by  their  mode  of  occurrence,  be  considered  as  parasites  on  certain  sub- 
terranean portions  of  the  material  of  massive  eruptions,  which  still  possess  a  high  tem- 
perature and  are  kept  in  a  liquid  state  by  the  molecular  combination  with  water  which 
finds  access  to  them.  This  mode  of  origin  of  volcanoes,  however,  is  only  a  repetition 
on  a  small  scale  of  the  manner  in  which  massive  eruptions  themselves  originated,  inas- 
much as  volcanoes  bear  a  similar  relation  to  the  latter  as  these  do  to  the  primeval  sub- 
stance composing  the  interior  of  the  globe,  to  which  the  fractures  descend.  These 
main  fissures,  which  are  probably  very  few  in  number  in  every  volcanic  belt,  form  the 
great  arteries  in  this  harmonious  system.  Their  common  origin  will  furnish  an  ex- 
planation of  the  general  similarity  of  the  phenomena  presented  by  different  volcanic 
belts  ;  while  the  varied  and  possibly  very  intricate  mode  of  their  ramifications  towards 
the  surface,  together  with  th'e  different  conditions  of  the  rocks  which  they  intersect,  the 
various  rate  and  local  diversity  of  the  access  of  water,  the  different  circumstances  which 
may  determine  the  depth  in  which  the  expansive  force  of  vapor  can  be  brought  into 
action  (among  these  may  be  the  relative  proportion  in  which  chlorine,  fluorine  and  sul- 
phur are  present),  and  other  influences  unknown,  would  give  ample  means  for  explain- 
ing the  diversity  of  all  the  phenomena  of  vulcanism  within  each  separate  volcanic  belt: 
such  as  the  apparently  intricate,  and  yet  to  a  certain  degree  harmonious,  mode  of  dis- 
tribution of  the  rocks  ejected  ;  the  correlations  existing  among  the  latter  in  regard  to 
their  composition  ;  the  dependency  of  volcanic  action  on  massive  eruptions  ;  the  mode 
of  distribution  of  hot  springs,  solfataras,  geysers,  and  other  phenomena  which  were  ap- 
parently associated  with  both  modes  of  action  ;  the  different  phenomena  connected  with 
the  occurrence  of  earthquakes  ;  the  small  size  of  their  area  of  disturbance  when  con- 
nected with  volcanic  action,  its  varying,  and  sometimes  very  great  extent  in  other 
cases  where  no  connection  with  any  one  distinct  volcano  can  be  discovered  ;  the  singu- 
lar correlations,  finally,  which  have  been  observed  to  exist  between  different  volcanic 
vents  situated  on  the  same  belt.  Some  hints  may  even  be  got  in  regard  to  the  remoter 
correlations  which  apparently  exist  between  the  phases  of  volcanic  action  on  neighbor- 
ing belts,  though  it  must  be  conceded  that  in  respect  to  the  latter  numerous  facts  have 
been  observed  which  cannot  be  satisfactorily  explained  in  the  way  here  proposed,  and 
rather  appear  to  indicate  the  influence  of  the  phases  of  magnetic  currents  on  the 
manifestations  of  vulcanism.22 

w  An  extremely  valuable  contribution  to  the  elucidation  of  these  correlations  respecting  the  phases  of  volcanic  ac- 
tivity waa  lately  given  by  Dr.  Emil  Kluge,  in  his  work,  Ueber  den  Synchronisms  und  Antaganismus  von  vulcanischen 
Eruptionen,  Leipzig,  1 863.  The  clear  and  able  compilation  of  facts  will  be  of  lasting  interest,  though  grave  objections  may 
be  raised  against  the  author's  views  on  the  origin  of  volcanic  action,  which  are  not  based  on  any  inferences  drawn  from  the 
correlations  demonstrated  in  the  same  book. 

(104) 


OF   VOLCANIC   ROCKS.  67 

The  foregoing  considerations  may  explain  what  appears  to  have  been  the  most 
probable  way  in  which  volcanic  activity  was  developed  at  and  near  the  places  of  mass- 
ive eruptions.  They  apply  without  difficulty  to  those  cases  where  volcanoes  have  un- 
dergone no  change  in  regard  to  the  character  of  their  lava.  We  have  still  to  consider 
those  more  intricate  cases  where  the  latter  has  periodically  changed.  There  are  volca- 
noes which  exhibit  a  regular  succession  of  andesitic,  trachytic,  and  rhyolitic  lavas,  fol- 
lowed in  a  later  period  by  the  outpouring  of  basaltic  lava,  either  through  the  same,  or 
through  other  vents  in  the  immediate  vicinity.  There  are  others  in  which  only  a  part 
of  this  series  can  be  observed,  such  as  the  succession  of  basalt  to  trachyte  or  rhyolite, 
which  appears  to  be  of  the  most  frequent  occurrence,  or  of  rhyolite  only  to  andesite. 
As  the  order  of  succession  is  generally  the  same  as  that  exhibited  by  massive  eruptions, 
it  would  appear  that  it  must  be  due  to  the  same  causes  in  both  cases.  If  we  take  it 
for  granted,  that  those  extensive  reservoirs  of  melted  matter,  from  which  was  either 
continued  its  quiet  protrusion,  or  volcanoes  were  fed,  have  had  their  seat  in  those  con- 
centric layers  of  the  crust  composed  of  silicious  masses  which  correspond  to  trachyte 
or  rhyolite  in  composition,  then  there  is  little  difficulty  in  explaining  the  protrusion 
of  portions  of  them.  For  there  must  be  a  limit  to  the  expansion  of  a  substance  such 
as  a  given  mass  of  andesite,  by  aqueous  fusion,  and  thereby  a  limit  to  its  ejection. 
The  eruptive  activity  might  then  either  come  to  rest  or  continue.  As  it  is  very  im- 
probable that  water  takes  originally  a  part  in  the  composition  of  those  masses  below 
the  shell  of  sediments  which  have  crystallized  from  a  molten  state,  its  access  to  them 
at  places  contiguous  to  a  source  of  heat,  such  as  must  be  given  by  a  fissure  filled  with 
molten  matter  from  below,  must  be  attended  by  a  powerful  influence  on  them.  It 
would  exert  itself  in  aqueous  fusion  and  expansion.  But  the  viscidity  peculiar  to  these 
highly  silicious  substances  would  not  allow  their  extrusion  until  after  the  more  liquid 
audesitic  masses  had  been  ejected.  Supposing  the  reservoir  in  which  this  first 
change  was  effected  to  have  been  in  the  trachytic  region,  further  action  from  the  same 
could  be  cut  off  by  the  cessation  of  the  ingress  of  water  to  it.  Another  reservoir,  sit- 
uated in  the  rhyolitic  region,  might  then  be  isolated  within  the  educting  channel,  filled 
now  with  trachytic  matter,  and  the  same  process  repeated,  as  before,  ending  with  the 
change  of  trachytic  into  rhyolitic  rocks.  As  regards  the  succession  of  basalt  to  these 
silicious  rocks,  we  refer  to  the  fact  established  before,  that  the  fissures  which  gave  vent 
to  basalt,  have  all  been  formed  at  a  much  later  epoch  than  those  through  which  andesite 
had  ascended,  and  that  they  were  only  partly  coinciding  with  them.  It  would  appear 
that  subterranean  reservoirs  of  liquid  matter,  connected  with  the  surface  by  channels  of 
ejection,  should  have  offered,  in  many  cases,  the  places  of  least  resistance.  It  may, 
therefore,  be  inferred,  that  basalt,  the  great  comparative  liquidity  of  which  is  a  well- 
known  fact,  would  enter  many  of  those  reservoirs,  and  be  emitted  through  the  same, 
or  through  newly-formed  channels,  in  preference  to  any  matter  of  a  more  viscous  con- 
sistency ;  and  it  will  not  be  difficult  to  understand  why  basalt  should,  in  many  instances, 
have  again  been  followed  by,  or  alternated  during  long  epochs  with,  lavas  of  a  rhyo- 
litic composition. 

These  considerations,  which  may  be  equally  applied  to  the  order  of  succession 
of  massive  eruptions  and  to  that  of  volcanic  lavas,  are  not  given  with  a  view  of  cxplain- 

(105) 


68  RICHTHOFEN NATURAL   SYSTEM 

ing  this  intricate  subject,  but  only  to  show  that  natural  occurrences  may,  with  the  aid 
of  the  theory  here  advocated,  be  explained  without  having  recourse  to  any  forced  as- 
sumptions. There  is  no  one  of  the  processes  pointed  out  which  is  not  within  the  limits 
of  those  we  are  accustomed  to  consider  as  highly  probable  in  regard  to  that  part  of 
volcanic  action  which  is  removed  only  a  little  way  beyond  immediate  observation,  and 
therefore  more  accessible  to  well-founded  speculation  than  is  the  remoter  connection 
between  volcanic  action  and  the  fissures  through  which  the  massive  eruptions  of  vol- 
canic rocks  took  place. 

3.    Other  Tlieories  respecting  the  Origin  of  Eruptive  Rocks. 

The  various  theories  which  have  been  proposed  in  regard  to  the  origin,  not 
only  of  the  volcanic  but  of  all  those  non-foliated  crystalline  rocks  which  are  made  up 
of  silicates,  diverge  in  different  directions.  Most  of  them,  however,  leave  unnoticed 
the  most  essential  features  of  those  rocks,  such  as  their  nature  in  regard  to  the  details 
of  chemical  composition,  their  similarity  in  character  in  distant  countries  and  different 
ages,  the  laws  of  their  mode  of  succession  and  distribution,  and  the  fact  of  their  peri- 
odical emission  after  long  periods  of  repose  ;  and  no  one  undertakes  to  account  for 
all  of  them.  There  may  be  distinguished  two  classes  of  these  theories  :  the  first  com- 
prehends those  which  assume  the  original  seat  of  eruptive  rocks  to  have  been  beneath 
the  sedimentary  rocks,  while  the  second  embraces  those  which  would  have  it  to  be 
within  the  shell  composed  of  the  latter.  It  was  the  purpose  of  our  foregoing  theoret- 
ical considerations  to  point  out,  that  it  is  exclusively  in  the  direction  followed  by  the 
theories  of  the  first  class  that  we  may  at  all  look  for  a  satisfactory  explanation  of  the 
relations  presented  by  the  eruptive  rocks.  But,  though  the  views  here  advocated 
belong  altogether  to  this  class,  the  leading  theories  embraced  in  it  have  a  very  different 
scope.  That  form  of  them  which  was  held  by  Buch,  Humboldt  and  others  of  the  most 
prominent  geologists,  and  is  still  quite  largely  adopted,  starts  from  the  assumption  that  all 
eruptive  rocks  were  ejected  in  the  same  condition  in  which  they  are  supposed  to  have 
been  when  at  their  original  place  in  the  earth's  interior,  that  is,  molten  by  dry  heat  ; 
while  the  contraction  of  the  globe  by  loss  of  heat  is  regarded  as  the  sole  cause  of  their 
ejection.  Among  the  weighty  objections  which  may  be  raised  against  these  theories, 
may  be  mentioned  :  that  the  eruptive  rocks  on  their  arrival  at  the  surface  have  evi- 
dently not  had  a  temperature  which  would  be  sufficient  for  their  dry  melting  ;  that 
they  contain  a  certain  proportion  of  water  enclosed,  which  was  formerly  not  brought 
into  account  ;  that  the  ejection  from  the  depth  to  the  surface  of  masses  molten  by  dry 
heat  is  a  process  impossible  of  explanation,  and  that,  if  it  was  possible,  the  rocks 
should  have  a  different  texture  from  that  exhibited  by  granite,  diorite  or  propylite  ; 
that,  finally,  contraction  alone  is  as  little  capable  of  furnishing  an  agent  for  the  rending 
of  fissures  opening  towards  the  surface,  which  is  the  prime  condition  of  eruptive 
activity,  as  is  the  cooling  of  the  globe  of  giving  the  conditions  requisite  for  the  process 
of  ejection  itself.  The  theories  mentioned  have  been  longest  maintained  on  the 
European  continent,  where  they  are  even  now  advocated  by  many.  Though  approach- 
ing nearest  of  all  to  the  most  probable  mode  of  origin  of  eruptive  rocks,  the  reasons 

(106) 


OF   VOLCANIC    ROCKS.  69 

why  they  are  untenable,  with  our  present  state  of  knowledge,  are  too  obvious  to  be 
made  an  object  of  a  more  detailed  explanation. 

More  numerous  and  more  obvious  objections  may  be  raised,  chiefly  from  a  geo- 
logical point  of  view,  against  those  theories  of  the  second  class,  according  to  which  all 
the  rocks  under  consideration,  with  the  exception  of  those  lavas  which  we  actually  see 
being  ejected  from  volcanoes,  would  have  derived  their  origin  exclusively  from  meta- 
morphism  in  situ  ;  be  it  that  the  change  is  supposed  to  have  been  mainly  of  a  chemical 
nature,  and  effected  on  or  beneath  the  surface,  by  the  action  of  water  alone  containing 
certain  substances  in  solution,  or  that  refuge  be  taken  to  the  cooperation  of  super- 
heated water  and  pressure  at  a  great  depth.  A  large  amount  of  positive  facts  as  well 
as  of  sagacious  reasoning  have  been  applied  in  their  defense  ;  but  viewed  in  the  light 
of  those  observations  which  present  themselves  continually  to  the  geologist'  in  the 
field,  in  evidence  of  an  essentially  intrusive  and  extrusive  nature  of  those  rocks,  the 
premises  on  which  argumentation  is  based  must  appear  extremely  deficient.  Also 
will  the  reasons  which  we  shall  adduce  against  the  leading  theory  of  our  day,  be  appli- 
cable a  fortiori  against  the  assumption  of  an  origin  of  our  "eruptive"  rocks  only  by 
metamorphism  in  situ.  Before  entering  upon  that  leading  theory,  we  have  still  to 
mention  the  existence  of  a  number  of  others,  which,  though  acknowledging  the  proba- 
bility of  an  oi-igin  of  all  lava  and  "  trap-rocks"  from  the  liquid  interior  of  the  globe, 
assume  that  granite,  syenite,  diabase,  diorite  and  certain  porphyries  are  so-called  hypo- 
gene  rocks,  that  is,  have  originated  by  metamorphism  of  sediments  in  situ.  Against 
these  theories  may  be  raised  the  collective  objections  which  apply  severally  to  the 
others. 

The  obvious  objections  which  may  be  made  to  the  theories  hitherto  mentioned 
have  given  more  and  more  ascendency  to  another  doctrine,  which  we  may  designate 
as  the  metamorphic  theory  of  eruptive  rocks,  and  which  owes  its  great  influence  upon 
modern  geology  to  the  fact  of  its  starting  from  a  certain  number  of  established  geo- 
logical facts  and  lithological  observations,  and  from  the  results  of  experiments.  It  is 
eminently  a  theory  of  the  second  class.  No  arguments  against  it  will  be  more  potent 
than  those  which  prove  the  fallacy  of  the  basis  of  all  the  theories  of  this  class,  as 
they  will  show  that  we  must  look  for  the  origin  of  eruptive  rocks  altogether  in  a  dif- 
ferent direction  from  that  followed  by  them. 

The  metamorphic  theory  is  essentially  to  the  purport,  that  all  eruptive  rocks, 
whether  of  recent  or  of  ancient  age,  whether  ejected  by  volcanic  action  or  carried  into 
their  present  position  without  any  sign  of  the  latter,  were  originally  sedimentary  rocks 
rendered  liquid  by  the  cooperation  of  heat,  pressure,  and  water.  It  is  supposed  that 
these  rocks,  by  the  continued  superposition  of  immense  masses  of  sediment,  had  ar- 
rived at  a  great  depth  under  the  surface,  where  the  agencies  mentioned  would  cooper- 
ate to  modify  and  transform  their  state  of  aggregation,  resulting  either  in  a  molecular 
change  alone,  or  in  their  conduction  into  a  state  of  fusion.  In  the  former  case,  the 
sediments  would  be  simply  metamorphosed,  while  in  the  latter,  they  would  either 
crystallize  in  depth,  with  a  total  loss  of  their  original  structure,  and  form  "plutonic," 
or  "hypogene,"  or  "indigenous  "  rocks,  or  be  forced  upwards  through  fractures,  and 
solidify  partly  in  the  conducting  channels  and  partly  on  the  surface,  when  they  would 

N  (107) 


70  R1CHTHOFEN NATURAL   SYSTEM 

form  "  trap  rocks  "  and  "lava."  One  common  cause  is  thus  assigned  to  both  meta- 
morphic  and  eruptive  (including  volcanic)  action,  the  latter  being  considered  an  ad- 
vanced stage  and  ultimate  result  of  the  former.  The  immense  action  of  metamorphism 
is  an  undeniable  fact.  But  while  formerly,  on  account  of  the  violent  agencies  which 
were  supposed  to  have  been  required  for  it,  and  the  growing  conviction  that  heat  alone 
could  not  produce  effects  on  such  an  enormous  scale  as  had  been  suggested,  the  sub- 
ject had  to  be  treated  with  caution,  and  any  extreme  assumption  was  received  with 
doubt,  if  not  with  a  certain  repulsion,  the  condition  of  things,  in  this  respect,  has  of 
late  undergone  a  great  change,  as  scientific  experiments,  and  especially  those  of  Dau- 
bree,  have  demonstrated  the  extent  of  the  influence  of  water  and  pressure  in  producing 
metamorphic  action.  They  have  proved  the  remarkable  fact  that,  at  a  comparatively 
low  temperature,  and  with  the  aid  of  pressure,  the  effects  of  water,  if  continued  for  a 
sufficient  time,  particularly  when  it  is  charged  with  alkaline  substances,  will  be  able 
to  produce  changes  in  the  nature  of  rocks  which  surpass  even  the  most  audacious 
assumptions  of  former  time.  An  apparently  safe  foundation  was  now  given  to  the 
widest  generalizations,  and  the  consequence  is,  that  an  almost  unlimited  action  is  at  the 
present  time  brought  to  the  account  of  metamorphism.23  The  least  founded  concep- 
tion, however,  of  the  faculties  ascribed  to  it,  we  consider  to  be  its  supposed  sole  in- 
strumentality in  the  production  of  volcanoes  and  the  eruptive  action  of  former  ages 
from  portions  of  the  shell  of  sedimentary  rocks.  It  is  true  that  a  great  additional 
degree  of  apparent  probability  has  been  given  to  the  metamorphic  theory  of  eruptive 
rocks  by  the  results  of  the  microscopic  examination  of  rocks  so  successfully  instituted 
by  Sorby,  since  they  prove  a  remarkable  similarity,  in  the  minutest  texture,  of  the 
minerals  constituting  granite  and  some  cognate  non-foliated  rocks,  with  the  main  in- 
gredients of  certain  foliated  rocks  made  up  of  silicates,  such  as  gneiss  and  micaschist. 
These  had  been  observed  long  before  to  form  the  last  link  of  a  series  which  may  be 
traced,  by  slow  gradations,  from  unmetamorphosed  sedimentary  rocks,  through  those 
which  are  undoubtedly  metamorphosed,  up  to  the  crystalline  foliated  rocks  mentioned. 
The  newly  discovered  facts  appeared  to  indicate  that  granitic  texture  is  the  most  ad- 
vanced stage  in  a  progressive  series  of  molecular  changes,  and,  carrying  the  argumenta- 
tion still  farther,  the  assumption  was  apparently  justified  that  a  corresponding  origin 
must  be  ascribed  to  other  rocks,  such  as  those  of  volcanic  origin,  which  are  connected 
with  granite  by  another  long  and  gradual  line  of  passage. 

In  drawing  up  our  argument  against  this  theory,  (including  all  those  doctrines 

23  It  may  be  frequently  noticed,  that  those  eminent  men  who,  by  creating  a  firm  basis  for  induction,  have  pointed 
out  the  path  through  regions  in  the  field  of  science  the  knowledge  of  which  had  consisted  before  of  a  confused  accumula- 
tion of  facts  and  suggestions,  did  themselves  apply  the  newly  acquired  views  within  moderate  limits,  while  sweeping  general- 
izations on  the  same  basis  were  usually  made  by  others.  It  is  so  in  the  present  case.  The  limits  within  which  Daubrde 
himself  has  applied  his  ingenious  conclusions  from  his  experiments  on  the  action  of  superheated  water  and  pressure  upon 
silicates,  will  probably  never  be  drawn  any  closer.  No  discovery,  however,  could  have  been  more  opportune  to  those  who 
had  advocated  before  the  origin  of  eruptive  by  the  remelting  of  sedimentary  rocks.  Against  the  form  of  this  theory  which 
was  first  proposed  by  Hntton,  and  enlarged  by  Lyell,  Babbage,  Herschel  and  others,  similar  objections  could  be  raised  as 
against  that  form  of  the  theories  of  the  first  class  which  was  held  by  Buch.  With  the  aid  of  the  new  views  acquired, 
however,  it  was  remodeled  and  brought  into  its  present  shape.  A  far  greater  extent  is  thus  being  given  to  the  conclusions 
from  Mr.  DaubreYs  experiments  than  their  author  ever  intended. 

(108) 


OF   VOLCANIC   ROCKS.  71 

which  assume  either  all  or  a  part  of  the  massive  crystalline  rocks  composed  of  silicates 
to  have  originated  by  the  metamorphism  of  sediments)  we  will  first  point  out  some 
reasons  against  its  general  tenor,  which  may  be  conclusive  in  regard  to  those  rocks 
the  eruptive  nature  of  which  is  almost  generally  conceded,  while  we  will  have  to 
bring  some  additional  arguments  against  its  special  application  to  those  which  are  often 
designated  as  "  hypogeue  "  or  "  plutonic." 

The  metamorphic  theory  starts  from  the  assumption  of  an  alternating  progres- 
sion from  the  center  of  the  earth,  and  recession  towards  it,  of  the  chthonisothermal 
planes,  the  former  being  caused  by  sedimentary  deposition,  the  latter  by  denudation.24 
It  is  then  argued  that,  by  the  progression  of  these  planes,  sedimentary  strata  would 
acquire  a  more  and  more  elevated  temperature,  and,  being  permeated  by  water,  would 
be  metamorphosed,  and  finally  rendered  liquid.  It  is  demonstrated  that  heat,  generated 
by  the  plication  of  the  strata  in  the  lowest  and  central  part  of  an  area  of  subsidence, 
would  aid  in  promoting  these  changes,  which  would  end  in  the  rupturing  of  the  crust 
and  the  protrusion  of  liquid  matter.  Eruptive  activity  should,  according  to  these 
views,  be  confined  to  areas  of  subsidence,  in  particular  to  their  central  portions.  Geo- 
logical observation  does  not  favor  this  conclusion,  since  the  emission,  at  least  of  the 
volcanic  rocks,  has  taken  place  on  the  borders  of  those  areas,  on  high  table  lands,  and, 
in  general,  in  places  which  have  undergone  elevation  before  and  since  the  time  of  the 
first  commencement  of  the  eruptive  activity. 

It  would  appear  that  the  experiments  of  Daubree  should  not  be  too  freely 
applied  to  reasoning  on  processes  within  the  shell  of  sedimentary  rocks.  They  have 
been  made  with  relatively  large  quantities  of  water,-  such  as  can  scarcely  be  expected 
to  be  present  in  solid  rock  at  some  distance  below  the  ground.  Supposing  that  that 
quantity  naturally  enclosed  in  it  would,  under  great  pressure,  cause  its  fusion,  then 
no  reason  can  be  adduced  why  there  should  not  prevail  a  liquid  state  of  all  matter  at  a 
limited  depth  below  the  surface,  and  over  extensive  regions,  if  not  over  the  whole 
globe.  That  such  is  not  the  case,  is  evident  from  the  want  of  any  signs  of  subterranean 
tides.  Leaving  this  difficulty  out  of  consideration,  another  presents  itself  concerning 
the  periodicity  of  eruptive  activity.  If  processes  such  as  those  suggested  by  the 
adherents  of  the  metamorphic  doctrine  were  its  cause,  then  it  would  be  impossible  to 
give  an  explanation,  why  there  have  been  long  eras  of  rest  intervening  between  others 
of  violent  eruptive  activity,  or  why  the  latter  was  of  general  distribution  over  the  globe 
during  the  Tertiary  era,  and  preceded  by  a  period  of  rest  which  was  probably  no  less 
general.  The  phenomena  of  vulcanism  might  have  manifested  themselves  at  a  certain 
time  more  in  one  country  than  in  another,  if  metamorphism  had  been  their  cause,  since 
their  principal  theater  would  be  constantly  shifted  to  the  places  of  the  most  violent 
metamorphic  action  ;  yet  they  should,  at  least  in  the  aggregate,  have  been  continuous. 

These  objections,  however,  against  the  metamorphic  theory  of  eruptive  rocks 
are  of  little  weight  when  compared  with  another,  respecting  their  nature  as  chemical 

24  It  is  needless  to  enter  here  upon  a  discussion  of  the  causes  and  effects  attributed  to  these  phases  in  the  "  flow  of 
heat  "  by  the  adherents  of  the  metamorphic  theory,  since  this  topic  has  been  made  lately  the  subject  of  a  paper  by  the  skillful 
hand  of  Professor  Dana. 

(109) 


72  RICHTHOFEN  —  NATURAL    SYSTEM 

compounds.  It  is  from  this  point  of  view,  as  we  have  repeatedly  remarked,  that  are 
offered  the  chief  and  most  deeply-founded  differences  between  the  eruptive  and  the 
stratified  and  foliated  rocks.  Definite  numerical  relations  on  the  one  side,  complete  ab- 
sence of  them  on  the  other,  and  a  transition  between  both,  marked  by  the  gradual  dis- 
appearance of  those  relations  with  the  passage  from  granite,  in  which  they  are  very 
distinct,  to  gneiss  and  micaschist — these  are  in  short  the  prominent  characters  of  the 
great  divisions  of  those  rocks  which  are  accessible  to  the  observations  of  the  geologist. 
We  alluded  at  another  place  to  the  corollary  of  this  distinction,  namely  :  that  it  is 
impossible  that  eruptive  rocks  are  remelted  sediments,  because,  if  they  were,  they 
would  necessarily  have  to  participate  in  the  varied  and  indefinite  chemical  composition 
of  these.  This  important  argument  appears  to  have  been  completely  overlooked  by 
the  adherents  of  the  metamorphic  doctrine,  and  it  would  be  sufficient  by  itself  to 
make  the  latter  appear  to  be  in  contradiction  with  the  true  state  of  facts.  K 

If  we  descend  from  this  general  tie  embracing  the  totality  of  eruptive  rocks, 
to  those  relations  which  either  separate  or  connect  distinct  groups  of  them,  in  regard 
to  the  time  at  which  they  came  to  the  surface,  we  are  unable  to  find  any  explanation 
of  the  peculiar  uniformity  of  these  relations,  if  we  adopt  the  metamorphic  doctrine. 
Rocks  of  great  mutual  similarity  might  have  been  occasionally  ejected  at  different 
places ;  but  the  repetition  of  the  same  order  of  succession  in  distant  regions  would  be 
just  as  inexplicable,  and  contradictory  to  the  nature  of  sedimentary  rocks,  as  the  per- 
fect chemical  identity  of  rocks  which  are  widely  separated  in  space,  but  occupy  a 
similar  position  in  the  order  of  succession. 

Another  geological  consideration  may  be  mentioned  which  appears  to  weaken  the 
metamorphic  theory.  It  is  known  that  the  most  ancient  rocks  are  distinguished,  in 
general,  by  a  much  greater  similarity  among  each  other  in  respect  to  chemical  compo- 
sition than  is  the  case  with  those  of  more  recent  origin,  and  that  one  prominent  feat- 
ure of  the  majority  of  them  is  the  presence  of  silica  in  a  similar  proportion  to  that  in 
which  it  is  contained  in  granite.  The  formation  of  sedimentary  rocks  having  been 
due,  at  all  times,  to  the  disintegration  of  rocky  matter  antecedent  to  them  in  age,  it  is 
obvious  that  ancient  sediments  would  have  participated  in  the  silicious  nature  of  the 

25  It  can  hardly  be  comprehended  that  it  should  have  been  maintained,  and  be  believed,  with  our  present  state  of  knowl- 
edge, that  clayslutes  were,  by  metamorphic  action,  converted  into  granite  and  syenite,  and  sandstones  into  porphyry  ;  since 
granite  and  (quartzose)  porphyry  are  chemically  identical,  while  clayslatc  and  sandstone  differ  in  this  respect  not  only  among 
themselves,  but  each  of  them  represents  a  large  number  of  different  accidental  compounds,  without  any  law  of  mutual  con- 
nection. The  supposition  that  "  the  presence  of  the  sandstone  formations  affords  the  conditions  required  for  the  occurrence 
of  the  great  porphyry-masses,"  gives  evidence  of  an  interpretation  of  natural  occurrences  by  preconceived  ideas.  There  are,  it 
is  true,  numerous  instances  of  sandstones  having  acquired,  by  metamorphic  action,  a  more  or  less  perfect  porphyritic  texture, 
so  as  to  be  hardly  capable  of  being  distinguished,  in  specimens,  from  true  eruptive  porphyries ;  but  geological  observation 
will  seldom  leave  any  doubt  in  regard  to  their  true  mode  of  occurrence ;  and,  as  a  generality,  it  is  not  difficult  to  distinguish 
these  metamorphic  rocks  with  porphyritic  texture  from  "  the  great  porphyry-masses,"  such  as  those  of  southern  Norway,  or 
middle  Germany,  to  which  the  above-mentioned  supposition  has  been  especially  applied.  It  is  evident  that  the  eruptions  of 
the  porphyritic  rocks  were  there  subaqueous,  and  that  the  true  relation  is  exactly  the  reverse  of  that  suggested  :  the  ejec- 
tion of  great  masses  of  qnartzose  porphyry  afforded  all  the  conditions  required  for  the  formation  of  sandstone  beds.  Quartzose 
porphyry  did  overflow  vast  regions,  and,  by  its  immediate  disintegration  into  tufaceous  matter,  gave  origin  to  those  red 
sandstones  by  which  it  is  so  often  accompanied.  These  cover  the  porphyry  often  in  horizontal  beds,  and  bear  to  it  similar 
relations  of  gradual  passage  and  intcrstratification,  as  trachytic  tufa  does  to  trachyte,  where  the  eruptions  of  the  latter  were 
subaqueous. 

(110) 


OF   VOLCANIC   ROCKS.  73 

rocks  by  the  destruction  of  which  they  originated,  and  should  have  had  a  tendency 
towards  an  average  between  them  in  regard  to  chemical  composition.  If  such  sedi- 
ments were  afterwards  rendered  liquid  by  metamorphic  processes,  and  either  carried  to 
the  surface  as  eruptive  rocks,  or  solidified  beneath  it,  so  as  to  form  part  of  it  only  after 
ages  of  denudation — if  they  were  then  again  partially  disintegrated,  re-deposited  and 
reerupted,  and  so  forth,  down  to  our  time,  the  final  result  should  be  an  ever-increasing 
uniformity  in  composition  of  both  eruptive  matter  and  sediments.  It  may  be  objected, 
that  this  tendency  towards  uniformity  must  have  been  checked  by  the  formation 
of  chemical  sediments,  as  by  them  a  portion  of  certain  elements  was  eliminated,  and 
must  have  become  almost  irrecoverably  lost  for  further  participation  in  the  supposed 
revolving  process.  The  substances  prevailing  in  chemical  deposits  are  lime,  soda  and 
magnesia.  Recent  eruptive  rocks  should,  therefore,  besides  showing  a  uniformity  of 
composition,  be  somewhat  deficient  in  those  substances,  and  contain  in  proportion 
more  of  silica,  alumina  and  potassa.  The  facts  are  exactly  reverse.  Not  alone  has  the 
variety  of  eruptive  rocks  rather  been  constantly  increasing  ;  but,  among  those  most 
recent  in  origin,  such  are  greatly  predominant  as  contain  lime,  soda  and  magnesia 
in  larger  proportion  than  ancient  rocks,  while  they  are  relatively  deficient  in  silica, 
alumina  and  potassa.  We  arrive  by  this  last  line  of  argument  at  the  conclusion,  in 
the  first  place,  that  the  present  variety  of  rocks,  especially  of  those  of  eruptive  origin, 
can  in  no  way  be  explained  by,  but  is  contradictory  to,  the  assumption  that  they  orig- 
inated by  the  repeated  destruction  and  reformation  of  the  material  which  constituted 
the  surface  of  the  globe  in  the  most  ancient  time.  From  this  it  follows,  in  the  second 
place,  that,  even  supposing  that  the  theory  should  be  admissible  for  the  majority  of 
eruptive  rocks,  entirely  new  matter  must  have  come  repeatedly  to  the  surface,  in  order 
to  replace  the  constant  loss,  as  it  were,  of  substances  such  as  lime  and  soda.  Eruptive 
rocks  ascending  from  sources  beneath  the  deepest  sediments  are  the  only  means  imag- 
inable for  the  performance  of  this  function.  The  enormous  extent  to  which  matter 
must  have  been  supplied  from  that  source  is  especially  evident,  if  it  is  considered  that 
there  must  have  been  a  time  when  no  sediments  existed  on  the  face  of  the  globe,  and 
that  the  disintegration  of  preexisting  matter  is  the  prime  condition  to  their  formation. 
The  entire  mass  of  the  sediments  which  make  up  the  exterior  shell  must,  therefore, 
have  been  derived  from  the  destruction,  partly  of  the  original  crust  which  solidified  on 
the  globe,  and  partly  of  those  rocky  masses  which  protruded  through  that  crust  to  the 
surface.  It  is  eminently  probable  that  the  mass  of  matter  derived  from  the  latter 
source  exceeds  very  far  that  which  is  due  to  the  first.  This  consideration  shows 
that  it  is  hardly  possible  for  us  to  form  an  adequate  idea  of  the  vast  importance  which 
the  periodical  emission  of  rocky  matter  from  places  beneath  the  primordial  surface  must 
have  had  in  the  history  of  the  formation  of  the  crust  of  the  globe. 

We  may  thus  start  from  any  point  of  view  within  the  range  of  positive 
knowledge,  and  we  shall  find  that  there  is  not  one  line  of  argumentation  which  does 
not  go  to  show  that  the  doctrine  of  the  origin  of  eruptive  rocks  by  the  metamorphism 
of  sedimentary  matter  is  irreconcilable  with  the  most  prominent  established  facts.  Not 
only  does  it  fail  completely  to  explain  them,  but  with  most  of  them  it  is  in  obvious 
contradiction.  But  as  a  metamorphic  origin  has  been  proved  to  evidence  in  regard  to 

(ill) 


74  KICHTHOFEN NATURAL   SYSTEM 

certain  foliated  rocks,  and  as  it  appears  no  less  safe  to  conclude  that  those  rocks,  the 
intrusive  or  extrusive  character  of  which  is  generally  admitted,  originated  from  be- 
low the  sedimentary  rocks,  we  have  still  to  examine  where  is  the  limit  to  which  the 
theory  of  such  an  origin  may  be  safely  applied.  A  line  of  gradual  passages  connects 
the  foliated  rocks  with  gneiss,  and  through  it  with  granite,  and  another  line  of 
gradual  passages  can  be  traced  from  the  volcanic  through  the  porphyritic  to  the 
granitic,  and  through  them  again  to  the  gneissoid  rocks.  The  affinity  of  granite  and 
gneiss  may  therefore  be  said  to  be  the  central  point  from  which  all  other  lines  of 
passage  in  the  nature  of  rocks  diverge  ;  and  it  must  be  ascribed  to  this  relation, 
that  many  geologists,  while  admitting  the  origin  from  below  the  crust  of  the  globe 
of  "trap-rocks"  and  "lava,"  still  consider  granite,  syenite  and  cognate  rocks  to 
have  been  generated  by  the  metamorphism  of  sediments,  and  to  occupy  now  the 
same  position  which  these  sediments  had  before  being  metamorphosed.  The  pe- 
culiar character  of  granitic  texture,  which  is  nearer  allied  to  that  of  foliated  crystal- 
line than  to  that  of  volcanic  rocks,  the  geological  occurrence  of  granite  in  intimate 
connection  with  those  and  its  apparent  independence  of  position  in  reference  to  the 
latter,  the  singular  order  of  solidification  of  the  minerals  constituting  granitic 
rocks — these  and  other  reasons  of  a  similar  kind  would  indeed  appear  to  support 
the  assumption  that  granitic  and  volcanic  rocks  differ  in  regard  to  their  origin  ;  while. 
on  the  other  hand,  additional  probability  is  apparently  given  to  the  connection  of 
granite  and  foliated  rocks  in  regard  to  their  mode  of  origin,  if  it  is  taken  into  con- 
sideration that  many  varieties  of  gneiss  resemble  granite  closely  in  chemical  com- 
position, although  others  deviate  from  it  in  this  respect  and  do  not  conform  to  the 
law  of  Bunsen. 

According  to  the  theory  here  advocated,  all  those  rocks  will  be  the  true  repre- 
sentatives of  the  primordial  mass  of  the  globe,  which  are  subject,  in  regard  to  their 
composition,  to  the  law  of  Bunsen,  and  occur,  besides,  in  positions  which  place 
beyond  doubt  their  intrusive  or  extrusive  origin,  so  far  as  they  no  longer  occupy 
their  original  place.  As  regards  the  first  distinguishing  mark,  it  is  known  that  granite, 
syenite,  diorite,  diabase,  and  nearly  all  those  porphyritic  rocks  which  have  been  con- 
sidered as  of  hypogene  origin  are  subject  to  the  law  of  Bunseu,  as  far  as  their  composi- 
tion has  been  ascertained.  In  this  respect,  therefore,  they  are  fundamentally  different 
from  sedimentary  and  metamorphic  rocks.  Although  believing  this  to  be  conclusive 
evidence  of  their  origin  from  below  the  shell  of  sediments,  we  must  enter  into  a  short 
discussion  of  the  chief  objections  which  have  been  raised  against  the  necessary  corollary 
from  this  mode  of  origin,  which  is  the  eruptive  character  of  those  portions  of  granite 
which  we  see  on  the  surface,  and  the  assumption  of  the  existence  of  this  as  the  foun- 
dation of  all  sedimentary  rocks. 

It  is  argued  by  the  adherents  of  the  metamorphic  theory  of  eruptive  rocks,  in 
the  first  place,  that  granite  cannot  be  the  fundamental  rock  in  the  crust  of  the  globe, 
on  which  those  of  sedimentary  origin  are  resting,  since  even  very  ancient  masses  of  it 
are  frequently  found  to  overlie  stratified  rocks.  But  this  fact,  which  was  known  long 
ago,  shows  only  that  it  is  impossible,  on  account  of  the  great  thickness  of  sedimentary 
rocks,  to  know  by  ocular  observation  of  what  nature  is  the  fundamental  rock.  It  is 
(112) 


OF    VOLCANIC    ROCKS.  75 

logically  absurd  to  assume  the  non-existence  of  a  foundation  consisting  of  rocks  differ- 
ing in  origin  from  sedimentary  rocks  ;  and  the  conclusion  is  inevitable,  that  it  must  be 
composed  of  rocks  which  were  generated  by  the  solidification  of  such  masses  as  formed 
part  of  the  primordial  substance  of  the  globe,  next  to  the  surface.  Probabilities 
accumulate  to  point  towards  the  assumption  that  very  silicious  granite  composes  that 
foundation,  together  with  such  gneissoid  rocks  as  must  be  supposed  to  have  been  formed 
in  an  incipient  sea  of  very  high  temperature,  and  under  the  pressure  of  the  superin- 
cumbent atmosphere  of  aqueous  vapors,  at  a  time  when  the  cooling  of  the  globe  had 
advanced  far  enough  to  allow  of  their  first  condensation,  and  when  pressure,  water, 
and  heat  could  cooperate  to  produce  on  its  surface  similar  effects  to  those  which  the 
same  agencies  are  supposed  to  have  wrought  in  later  periods  at  an  ever-growing  dis- 
tance from  it.  The  most  probable  mode  of  these  ancient  processes  has  been  pointed 
out  by  Daubree.  The  true  nature  of  the  foundation  rock  can,  of  course,  not  be 
positively  known,  but  must  remain  a  matter  of  conjecture.  No  solution  of  this 
problem  will,  however,  be  more  satisfactory  than  that  which  is  based  on  the  hypoth- 
esis of  Sartorius,  because  it  is  in  harmony  with  all  the  phenomena  of  vulcanism. 
The  probabilities  in  favor  of  a  granitic  foundation  are,  therefore,  very  great,  while 
no  valid  objection  has  yet  been  raised  against  its  having  the  nature  indicated. 

It  is  further  argued  that,  the  position  of  granite  being  always  in  the  midst  of 
sedimentary,  or  of  such  foliated  crystalline  rocks  as  are  connected  with  the  former  by 
gradual  passage,  and  often  conspicuously  above  such  rocks,  the  only  material  from 
which  it  could  be  generated  are  the  sedimentary  rocks  themselves.  The  arguments 
brought  in  evidence  of  these  assertions  are  among  the  most  potent  which  can  be 
adduced  against  them.  Granite,  in  Norway,  is  superposed  on  Laurentian  rocks  turned 
upon  their  edges,  and,  as  no  channel  can  be  found  by  which  it  ascended,  it  is  argued 
that  it  must  have  been  generated  by  metamorphism  in  situ.  But  as,  on  the  other  hand, 
it  has  justly  been  remarked,  that  no  position  is  more  favorable  for  metamorphic  action 
than  that  of  upturned  strata,  it  is  difficult  to  comprehend  why  the  Laurentian  rocks, 
which  were  evidently  nearer  to  the  source  of  heat,  should  not  have  been  metamor- 
phosed in  a  much  higher  degree  than  the  overlying  granite.  The  same  objection  may 
be  made  in  other  instances,  as  in  the  case  of  the  Huronian  and  Laurentian  rocks  in 
Canada,  which  were  found  to  be  overlain  by  granite  thousands  of  feet  in  thickness. 
The  negative  evidence,  apparently  afforded  by  the  fact  that  no  channels  have  been  dis- 
covered through  which  the  granite  could  have  protruded,  is  of  very  slight  value,  if  it 
is  taken  into  consideration  how  rare  are  the  instances  where  the  conducting  channels 
can  be  seen,  in  the  case  of  overflows  of  volcanic  rocks,  or  even  in  that  of  currents  of 
lava.  There  are  a  few  more  instances  known  where  granite  can  be  distinctly  observed 
to  overlie  the  upturned  edges  of  stratified  rocks,  and  must,  therefore,  have  overflowed 
the  same  in  a  liquid  state.  In  some  other  cases,  as  in  those  of  the  extensive  granitic 
areas  of  Bessarabia  and  Western  Australia,  the  true  geological  position  of  granite 
escapes  observation  ;  while,  in. a  number  of  others,  it  has  the  appearance  of  an  intrusive 
mass. 

The    strongest    objection    which    may    be    made   to    the   eruptive   nature  of 
granite   relates   to  its  lithological  characters,  which  are   different  from   those   of  the 

(113) 


76  KICIITHOFEN  —  NATURAL   SYSTEM 

eruptive  rocks  of  the  present  era.  The  metamorphic  theory  suggests  that  enormous 
volumes  of  those  masses  which  it  supposes  to  have  been  rendered  liquid  by  meta- 
morphism,  and  small  portions  of  which  were  occasionally  emitted  to  the  surface,  did 
solidify  below  ground  with  a  total  loss  of  their  previous  structure,  and  may  arrive  at 
the  surface  by  elevation  and  denudation.  Granite  is  supposed  to  have  been  in  all 
cases  a  hypogene  rock,  generated  by  metamorphism  in  those  same  positions  in  which 
it  is  found  at  the  present  time,  and  to  be  still  formed  continually  in  the  same  way  ; 
though  it  is  admitted  that  the  tension  which  may  have  attended  its  former  state  of 
liquidity,  may  have  caused  it  to  ramify  into  rents  and  fissures  of  the  overlying  rocks. 
We  have,  of  course,  to  assume  the  existence  of  an  immensely  greater  amount/  of  hypo- 
gene  matter  corresponding  in  composition  to  all  the  different  eruptive  rocks,  but  buried 
so  deep  beneath  the  present  surface,  that,  possibly  with  the  exception  of  a  few  granite 
masses,  it  never  does,  nor  ever  will,  form  part  of  it.  We  have,  for  this  reason,  to  con- 
sider all  non-foliated  crystalline  rocks  made  up  of  silicates,  and  which  are  visible  on  the 
surface,  as  having  been  removed  far  from  their  original  seat,  and  therefore  as  being 
eruptive.  It  is  a  matter  of  course  that  only  a  very  small  portion  of  all  matter  protrud- 
ed from  the  depth  would  be  ejected  to  the  surface,  as  by  far  the  greater  portion  would 
solidify  within  the  channels  of  ejection.  Granite  occupies  frequently  the  latter  posi- 
tion quite  distinctly.  But  if  we  consider  those  large  accumulations  of  the  same  rock 
covering  hundreds  of  square  miles,  we  can  only  explain  them  in  two  ways.  Either 
they  must  occupy  their  primeval  position,  as  is  supposed  by  the  adherents  of  the 
metamorphic  doctrine,  or  they  must  have  been  ejected  and  spread  in  a  liquid  state 
over  the  underlying  rocks.  The  objections  against  the  first  supposition  are,  the  adapta- 
tion of  granite  to  the  law  of  Bunsen,  and  the  fact  already  noticed,  that  it  overlies 
the  upturned  edges  of  strata  ;  the  objection  against  the  other  is  the  peculiar  mineral 
character  of  granite.  But  we  should  always  bear  in  mind  that  our  knowledge  in  re- 
gard to  the  conditions  required  for  producing  any  certain  kind  of  mineral  character 
within  a  compound  of  silicates  of  any  certain  composition  is  very  limited,  as  is  illus- 
trated, among  others,  in  the  cases  of  propylite  or  dolerite  ;  and  the  inferences  based 
on  subjects  which  are  imperfectly  known  to  us  should,  in  geology,  always  be  made 
subordinate  to  those  which  we  can  positively  establish  by  observations.  It  is  not  on 
the  ground  of  the  latter  that  a  hypogene  origin  is  ascribed  to  all  granite,  but  on  the 
ground  of  its  mineral  characters.  It  is  said  that  granitic  and  volcanic  rocks  offer  quite 
generally  a  different  appearance,  and  that  the  texture  of  the  former  indicates  crystal- 
lization under  great  pressure,  while  the  volcanic  rocks  were  evidently  solidified  on  the 
surface.  As  regards  the  first  assertion,  such  rocks  only  should  be  compared  together 
as  have  a  similar  composition,  that  is,  granite  with  rhyolite,  or  diabase  and  augitic 
porphyry  with  basalt ;  not,  as  is  ordinarily  done,  granite  with  phonolite  or  basalt.  We 
referred  already  to  the  similarity  in  character  between  certain  varieties  of  rhyolite  and 
granite.  It  is  corroborated  by  the  microscopic  examination  of  these  rocks  by  Ferd. 
Zirkel,  from  whose  memoir26  I  quote  the  following  passages  :  "  Quartziferous  trachytic 

26  Dr.  Ferdinand  Zirkel,  Mikroskopische  Gesteinsstudien,  in  Sitzungsberichte  der  Kais.  Acad.  der  Wissenschaften  zu 
WVen,  DO/.  47,  1863. 

(114) 


OP    VOLCANIC   ROCKS.  77 

porphyry  (rhyolite)  cannot  be  distinguished  under  the  microscope  in  any  respect  what- 
ever from  felsitic  porphyry.  If  one  dare  not  doubt  the  eruptive  origin  of  the  former, 
one  cannot  but  acknowledge  the  same  mode  of  origin  in  regard  to  the  latter."  And 
again:  "Let  us  here  remark,  that  the  microscopic  structure  (texture)  of  trachytic 
quartz  does  not  differ  in  any  respect  from  that  of  granitic  quartz  ;  there  is  no  difference 
between  these  two  families  of  rocks  so  widely  separated  in  time,  either  as  regards  the 
number  or  the  appearance  of  the  water  and  glass  cavities."  Considering  the  second  of 
the  before-mentioned  assertions,  it  is  true  that  Mr.  Sorby  has,  by  his  sagacious  obser- 
vations, come  to  the  conclusion  that  granite  solidified  at  a  temperature  of  about  600° 
Pahr.,  and  that  the  various  granites  have  been  formed  under  a  pressure  equivalent  to 
a  depth  of  from  40,000  to  69,000  feet.  The  calculation  by  which  these  figures  were 
obtained,  is  made  upon  the  basis  of  the  proportion  between  the  bulk  of  the  water 
contained  in  the  microscopic  cavities,  and  the  size  of  the  vacuity,  the  latter  being  sup- 
posed to  indicate  that  the  water  had  formerly  filled  the  entire  cavity  and  contracted 
within  it  after  the  solidification  of  the  surrounding  rock.  The  laws  of  hydrostatic 
pressure,  however,  as  Daubree  justly  remarks,  are  in  such  a  case  not  applicable  in  the 
same  way  as  they  would  be  in  a  column  of  water  ascending  through  a  fissure ;  temper- 
ature and  pressure  may,  in  a  mass  of  rock  solidifying  from  a  viscous  state,  be  pre- 
served, as  in  a  closed  vessel,  to  within  a  few  feet  from  the  surface  ;  "it  is,  therefore, 
possible  that  many  processes,  such  as  the  crystallization  of  granite,  may  have  been 
going  on  under  pressure,  though  at  a  very  limited  depth."  Considering  that  a  mass 
of  granite,  after  the  solidification  of  its  exterior  portion,  will  indeed  be  enclosed  within 
walls  that  may  offer  a  strong  resistance  equivalent  to  a  great  pressure,  it  may  be  in- 
ferred that  it  will  assume,  in  crystallizing,  a  similar  texture  to  that  which  it  would 
have  obtained  if  the  same  process  had  been  going  on  at  a  great  depth  below  the 
surface.  It  can  hardly  be  suggested  what  must  have  been  the  texture  of  the  crust  of 
a  granitic  mass.  But  the  age  of  the  granite  is  sufficient  to  justify  the  conclusion  that 
the  crust  of  those  masses  which  solidified  on  the  surface  of  the  globe  must  have  been 
completely  abraded,  and  only  those  portions  be  preserved  to  observation  which  con- 
solidated under  considerable  pressure. 

Though  these  arguments  may  show  that  the  difference  in  the  character  of  a 
large  mass  of  rock  need  not  necessarily  be  proportionate  to  the  depth  under  the  sur- 
face of  the  globe  at  which  it  solidified,  and  that  granitic  and  volcanic  rocks  are  nearly 
related  as  regards  their  microscopic  texture  ;  yet,  the  conspicuous  external  differences 
between  these  two  classes  of  rocks  must  remain  a  problem  difficult  of  solution.  The 
proportionate  rapidity  with  which  volcanic  rocks,  on  account  of  their  usually  small 
volume,  must  have  cooled,  may  be  among  the  reasons.  But  it  is  not  the  only  one.  There 
are  differences  in  the  nature  of  rocks,  which  escape  our  present  means  of  explanation. 
Hornblende-propylite,  though  no  doubt  at  all  can  be  entertained  in  regard  to  its  sub- 
aerial  solidification,  has  completely  the  character  of  the  so-called  Plutonic  rocks,  and 
yet  lacks  their  supposed  principal  distinguishing  mark  :  crystallization  in  depth.  Con- 
sidering that  certain  varieties  of  propylite,  andesite,  and  trachyte  are  modifications  of 
the  same  group  of  chemical  compounds  ;  and,  though  having  been  solidified  on  the  sur- 
face of  the  globe  within  a  short  period,  yet  exhibit  marked  external  differences,  we  can- 
o  (115) 


78  BiCHTHOFEN — NATURAL    SYSTEM 

not  be  surprised  to  see,  that  other  rocks  composed  of  silicates,  which  are  so  widely  sep- 
arated in  time  as  is  the  case  with  granite  and  rhyolite,  should  offer  even  greater  differ- 
ences. Some  further  clue  to  a  better  knowledge  of  this  subject  may  be  expected  from 
the  study  of  the  nature  of  volcanic  rocks.  We  see  the  same  chemical  compound 
forming  highly  viscid  lava  in  one  volcanic  crater,  while  it  is  quite  liquid  in  another. 
There,  it  solidifies  to  dolerite  or  leucitophyre ;  here,  to  basalt.  Similar  influences 
appear  to  work  greater  differences  in  more  silicious  compounds.  We  should  therefore 
put  little  value  on  the  doubts  entertained  in  regard  to  the  solidification  of  granite,  as  of 
a  rock  ejected  on  the  surface  and  expanded  over  it,  until  we  are  better  acquainted 
with  the  causes  of  the  lithological  differences  among  volcanic  rocks ;  and  the  evidence 
regarding  the  mode  of  origin  of  granite  should,  till  then,  be  mainly  taken  from  its 
geological  occurrence  and  its  chemical  composition. 


RELATION   OF  THE  DISTRIBUTION  OF  THE  VOLCANIC  ROCKS  TO  THE  CONFIGURATION  OF 

THE  SURFACE  OF  THE  GLOBE. 

If  we  embrace  in  a  broad  review  all  those  relations  mentioned  in  the  foregoing 
chapters  which  regard  the  history  of  eruptive  action,  we  arrive  at  the  general  conclu- 
sion that,  in  reference  to  the  entire  globe,  it  is  one  homogeneous  and  harmonious 
whole,  and  has  been  attended  by  such  gradual  changes  only  as  were  necessarily 
occasioned  by  the  progress  of  the  physical  development  of  the  globe  itself,  while,  in 
regard  to  every  different  part,  it  presents  a  series  of  distinct  phases  intimately  con- 
nected by  mutual  relations.  It  would  be  a  subject  worthy  of  the  closest  investigation, 
to  trace  the  effect  which  the  events  of  these  phases  have  had  severally  upon  the  struc- 
ture and  the  configuration  of  any  separate  country.  A  comparison  of  these  effects 
as  they  are  manifested  in  different  regions  would  then  aid  in  establishing  some  of  the 
chief  causes  of  the  differences  of  structure  peculiar  to  each  ;  and  the  knowledge  of 
these  would  make  us  better  acquainted  with  the  laws  governing  the  evolution  of  the 
globe,  and  prepare  the  way  for  a  more  thorough  understanding  of  its  physical  geog- 
raphy. I  will  only  attempt  at  this  place  to  trace  the  effects  referred  to  in  regard  to 
the  last  of  the  phases  of  eruptive  activity,  the  only  one  which  was  nearly  contempo- 
raneous in  all  countries.  It  occurred  at  a  comparatively  recent  date,  and  we  can  view 
the  results  in  a  clearer  light  than  we  can  those  which  are  remote  in  time  and  partly 
obliterated  by  the  vast  changes  which  since  then  have  revolutionized  the  surface. 

We  have,  in  the  first  place,  to  trace  more  in  detail  than  we  have  done  before, 
the  peculiarities  of  the  distribution  of  volcanic  rocks.  It  has  been  observed  that  ac- 
tive volcanoes  are  chiefly  situated  along  the  lines  of  the  present  sea-coasts,  especially 
at  the  foot  of  mountain  ranges  parallel  to  them ;  or  that  they  follow  elevated  sub- 
marine ranges,  when  they  will  either  remain  submerged  beneath  the  sea,  or  protrude 
above  it,  forming  chains  of  islands.  Some  of  these  appear  to  mark  the  lines  of  high 
mountain  ranges  bounding  submarine  continents,  if  we  may  use  this  expression  for 
those  areas  of  a  shallow  sea-bottom  which  are  separated  from  others  of  great  depth 
by  wall-like  elevations,  as  are  indicated  for  instance  in  the  main  ranges  of  the  vol- 

(116) 


OF   VOLCANIC    ROCKS.  T9 

canoes  of  the  Indian  Archipelago.  Active  volcanoes  have  also  been  found  to  be  par- 
ticularly numerous  in  those  regions  where  the  narrow  terminations  of  two  continents 
verge  towards  connection,  as  is  the  case  in  Central  America,  between  Alaska  and 
Kamtschatka,  and  between  Australia  and  Farther  India.  The  mode  of  distribution  of 
extinct  volcanoes  has  been  little  investigated.  But  as  they  are  far  more  numerous 
than  active  vents,  and  as  volcanic  action  has  completely  ceased  in  extensive  regions,  a 
perfect  understanding  of  the  distribution  of  volcanic  activity  on  the  globe  can  only  be 
acquired  when  they  shall  be  fully  taken  into  consideration.  The  general  laws  of  their 
distribution  appear  not  to  differ  from  those  relating  to  active  volcanoes.  They,  too, 
have  been  evidently  dependent  upon  the  course  of  the  sea-coasts  which  existed  at  the 
time  of  their  activity,  and  were  in  many  places  quite  different  from  those  of  the  pres- 
ent epoch.  But  we  must  note,  besides,  this  additional  feature  of  extinct  volcanoes, 
that  among  the  theaters  of  their  grandest  and  most  extensive  displays  are  certain 
countries  which  were  formerly  covered  with  large  inland  basins  filled  with  salt  water, 
and  form  now  mostly  elevated  table-lands  on  which  may  still  be  seen  the  remnants  of 
those  former  inland  seas.  Volcanoes  so  situated  form  often  crowded  groups  occurring 
at  distances  of  more  than  five  hundred  miles  from  the  sea-coasts  which  existed  con- 
temporaneously with  their  activity.  To  them  belong  the  numerous  extinct  volcanoes 
on  the  plateau  between  the  Sierra  Nevada  and  the  Rocky  Mountains,  a  region  which 
has  probably  had  no  equal  in  regard  to  the  extent  in  which  volcanic  activity  has  taken 
place,  as  it  comprises  the  States  and  Territories  of  Oregon,  Idaho,  Montana,  Utah,  Col- 
orado, Nevada,  California.  Arizona,  New  Mexico,  and  continues  southerly  to  the  high 
lands  of  Mexico.  Other  volcanoes  of  high  table-lands  are  those  of  the  Mongolian 
Desert,  the  Thian-Shan,  Armenia  and,  probably,  eastern  Africa.  The  volcanoes  of  all 
these  regions  are  extinct,  with  the  exception  of  a  few  in  the  southern  part  of  the  pla- 
teau of  Mexico,  which  are  in  action,  and  some  others  which  still  exhibit  solfataric  phe- 
nomena. 

Interesting  as  would  be  a  map  embracing  all  active  and  extinct  volcanoes,  it 
would  give  only  the  outlines  of  the  distribution  of  those  larger  accumulations  of  volcanic 
rocks  which  were  produced  by  massive  eruptions,  and  have  ordinarily  attracted  little 
attention.27  Geological  maps  being  often  imperfect  as  regards  them,  it  is  not  yet  pos- 
sible to  distinguish  any  other  features  of  their  distribution  than  those  mentioned  in 
reference  to  volcanoes,  as  they  can  always  be  found  in  the  vicinity  of  these.  We  may 
however  add,  that  massive  accumulations  may  eventually  be  found  near  the  summits 
of  mountain  ranges,  where  volcanoes  do  ordinarily  not  occur. 

27  This  may  be  exemplified  in  regard  to  the  Andes  of  North  and  South  America.  Humboldt  (Cosmos,  vol.  iv) 
distinguishes  five  groups  of  volcanoes,  separated  by  non-volcanic  regions.  They  are  mentioned  by  him  as  follows  :  1st,  vol- 
canic group  of  Mexico,  nearly  five  hundred  miles  in  length,  but  occupying  scarcely  more  than  one  degree  of  latitude,  on 
account  of  its  direction  from  east  to  west;  2d,  three  hundred  and  fifty  miles  free  from  volcanoes;  3d,  group  of  Central 
America,  having  a  length  of  over  eight  hundred  miles ;  4th,  seven  hundred  and  thirty-five  miles  non-volcanic  ;  5th,  group  of 
New  Granada  and  Quito,  five  hundred  and  fifty-five  miles  in  extent ;  6th,  the  longest  interval  without  volcanoes,  being  1,133 
miles ;  7th,  group  of  volcanoes  of  Peru  and  Bolivia,  extending  over  five  hundred  miles  ;  8th,  a  non-volcanic  space  of  six 
hundred  and  twenty  miles  :9th,  the  group  of  Chile,  the  most  extensive  range  of  volcanoes  of  America,  being  1,143  miles  in 
length.  Summing  up,  Humboldt  takes  about  3,000  miles  to  be  the  aggregate  extent  in  length  of  the  volcanic  regions,  2,838 
miles  that  of  the  intermediate  spaces  which  are  free  from  volcanoes.  If  all  the  volcanic  rocks  occurring  in  the  Andes 

,(1 17) 


80  RICHTHOFEN — NATURAL   SYSTEM 

If  we  turn  our  attention  from  the  mode  of  the  geographical  to  that  of  the  geo- 
logical occurrence  of  volcanic  rocks,  we  may  first  consider  from  this  point  of  view  the 
relations  just  mentioned.  The  two  modes  of  distribution  which  are  most  conspicuous, 
namely,  on  the  foot  of  mountain  ranges  and  along  sea-coasts,  are  nearly  identical  in  a 
geological  aspect.  For  most,  if  not  all  of  those  ranges  the  flanks  or  vicinity  of  which 
are  distinguished  by  the  occurrence  of  volcanic  rocks  on  a  large  scale,  have  been  contig- 
uous to  sea-coasts  in  the  Tertiary  period,  or  they  are  so  at  present,  or  they  have  been 
so  in  the  intermediate  time.  This  is  true  of  active  and  extinct  volcanoes,  as  well  as  for 
massive  eruptions,  in  the  Carpathians,  on  the  southern  foot  of  the  Alps,  on  the  borders 
of  the  plains  of  northern  Germany,  on  the  slopes  of  the  central  plateau  of  Asia,  in  their 
whole  extent  from  Armenia  and  the  Caucasus,  passing  Lake  Issikul  and  Lake  Baikal, 
to  the  vicinity  of  Pekin,  in  the  coast  ranges  of  California,  in  the  Cascade  Mountains  of 
Oregon,  and  in  numerous  other  countries.  As  regards  the  prominent  occurrence  of 
volcanic  rocks  on  high  table-lands,  it  is  analogous  to  their  situation  on  sea-coasts,  inas- 
much as  the  salt  lakes  occurring  on  them,  which  were  of  much  larger  size  in  the  Ter- 
tiary period,  would  be  an  equivalent  of  the  vicinity  of  the  sea.  Those  larger  regions 
not  covered  by  salt  lakes,  which  are  situated  in  the  interior  of  continents,  and  have 
been  so  since  the  commencement  of  the  Tertiary  period,  were  generally  not  the  theater 
of  outbreaks  of  volcanic  rocks.  This  circumstance  renders  almost  certain  the  influence 
which  the  neighborhood  of  large  quantities  of  salt  water  has  had  upon  the  commence- 
ment and  main  phases  of  the  eruptive  activity,  though  its  continuation  in  the  latter 
phases,  which  are  those  of  volcanic  action  proper,  may  in  numerous  instances  have 
been  maintained  by  fresh  water. 

The  relations  of  the  distribution  of  the  volcanic  to  that  of  the  granitic  and  por- 
phyritic  rocks  have  been  considered  in  another  chapter.  There  is  probably  no  place  dis- 
tinguished by  the  accumulation  of  the  first  where  the  previous  eruption  either  of  granitic 
or  of  both  granitic  and  porphyritic  rocks  may  not  be  observed,  if  the  conditions  are  such 
that  observations  in  this  direction  are  possible.  But  as  there  are  porphyritic  regions  out- 
side of  the  volcanic  belts  (for  instance,  the  quartzose  porphyries  of  southern  Norway,  or 
the  Triassic  trap-rocks  of  the  Connecticut  Valley),  and  as  the  same  is  true  of  a  great 
number  of  known  granitic  districts,  and  probably  of  a  far  greater  number  of  others 
which  are  not  accessible  to  observation,  it  is  evident  that  the  existence  of  ancient  chan- 
nels of  ejection  did  not  necessarily  imply  their  reopening  in  the  Tertiary  period,  while 
there  can  scarcely  be  any  doubt  that  the  once  shattered  places  where  they  were  situ- 
ated offered  less  resistance  to  the  formation  of  new  fractures,  than  other  portions  of 
those  regions  in  which  the  greatest  disturbances  took  place  in  the  Tertiary  period. 

We  have,  finally,  to  mention  the  connection  which  apparently  exists  between  the 
occurrence  of  volcanic  rocks  and  the  regions  where  ancient  formations  have  been 


were  laid  down  on  a  map,  it  would  hardly  show  such  great  intermissions.  The  extensive  fields  of  lava  found  by  Capt. 
Fitzroy,  in  southern  Patagonia,  the  volcanic  rocks  of  the  Desert  of  Atacama,  and  of  northern  Peru,  are  instances  of  their 
extensive  distribution  in  the  non-volcanic  spaces  of  South  America,  while  on  the  northern  continent  they  are  one  of  the 
main  features  in  the  structure  of  the  great  western  mountain  ranges,  probably  throughout  tlirir  whole  extent  from  I'uuamu 
to  the  peninsula  of  Alaska,  anil  are  accompanied  probably  by  thousands  of  extinct  craters. 

(118) 


OF   VOLCANIC   KOCKS.  81 

highly  disturbed  and  subjected  to  an  extensive  metamorphism.  It  appears  that  the 
eruptive  action  was  in  a  great  measure,  perhaps  absolutely,  limited  to  regions  which 
offered  this  condition.  But  this  connection  by  no  means  justifies  the  conclusion  that 
the  existence  of  metamorphic  foliated  rocks  was  among  the  first  causes  of  the  ejection 
of  volcanic  rocks.  There  are  not  only  countries,  such  as  the  mountains  of  Scandinavia, 
the  Ural  Mountains,  the  Appalachians,  and  other  ranges  distinguished  by  the  records 
of  a  very  ancient  metamorphism,  where  no  trace  of  volcanoes  has  been  found  ;  but  this 
applies  even  to  a  few,  though  particularly  grand  instances  of  mountain  ranges  which 
were  the  theater  of  metamorphic  action  on  a  grand  scale  within  the  most  recent  periods, 
such  as  the  Alps,  the  Himalaya,  and  the  Pyrenees. 

The  two  last  named  relations,  both  conspicuous,  yet  both  secondary  in  import- 
ance, are  evidently  nearly  identical,  since  granite  enters,  probably  in  all  cases,  into  the 
structure  of  those  same  countries  which  have  been  the  theater  of  an  extensive  meta- 
morphic action.  It  is,  however,  worthy  of  note  that,  of  the  regions  so  distinguished, 
part  of  those  only  appear  to  have  become  the  theater  of  eruptive  activity  in  the  Ter- 
tiary period,  where  granite  must,  by  the  mode  of  its  occurrence,  be  assumed  to  have 
formerly  been  extruded  to  the  surface,  as  is  the  case  in  the  Sierra  Nevada,  on  the 
southern  foot  of  the  Alps,  in  middle  Germany,  and  in  central  France,  while  no  volcanic 
rocks  arrived  at  the  surface  in  other  mountain  ranges  where  granite  occurs  only  in  the 
shape  of  wedges  surrounded  by  foliated  rocks,  and  bears  a  merely  intrusive  character, 
as  in  the  central  range  of  the  Alps.28 

28  Another  peculiarity  in  the  distribution  of  volcanic  rocks  may  be  noticed.  It  is  their  outbreak  at  places  of  dis- 
location or  faulting  of  mountain  ranges,  if  we  may  apply  these  expressions  for  the  immense  displacements  which  some  of  the 
latter  have  undergone,  along  lines  either  at  angles  with  or  parallel  to  their  direction.  A  remarkable  instance  of  the  first 
kind  is  afforded  in  Hungary,  where  a  fracture,  along  the  line  Eperies-Kaschau,  crosses  the  Carpathians  in  a  nearly  meridional 
direction.  To  the  west  of  it,  ancient  formations,  along  with  granitic,  porphyritic,  and  volcanic  rocks,  make  up  the  surface, 
and  are  elevated  to  high  mountain  ranges  culminating  in  the  High  Tatra.  They  extend  eastward  close  to  the  fracture,  and 
there  terminate  abruptly.  East  of  it,  the  main  chain  of  the  Carpathians  continues  as  a  range  of  little  elevation,  and  con- 
sisting chiefly  of  Cretaceous  and  Kocene  rocks,  while  the  high  foothills  of  the  western  part  are  replaced  by  the  Hungarian 
plains.  Immediately  out  of  the  fracture  rises  the  Eperies-Kaschau  range,  of  nearly  one  hundred  miles  in  length,  which 
consists  exclusively  of  volcanic  rocks.  The  subsidence  of  the  land  on  the  eastern  side  of  the  fracture  must  have  amounted 
to  several  thousand  feet  in  this  case.  A  transverse  dislocation  of  similar  magnitude,  by  which  the  western  side  was  sunk 
thousands  of  feet,  is  observable  on  the  eastern  bank  of  the  Rhine,  along  the  boundary  of  Switzerland  and  Vorarlberg.  No 
volcanic  rocks  however  are  visible  at  that  place,  which  fact  appears  to  be  in  accordance  with  the  absence  of  any  ancient 
eruptive  rocks  in  the  neighborhood,  and  the  wedge-like  shape  of  granite  in  the  nearest  places  of  its  occurrence.  A  peculiar 
instance  is  afforded  by  the  three  porphyritic  regions  on  the  southern  slope  of  the  Alps,  each  of  which  indicates  a  deep  de- 
pression, bounded  by  longitudinal  and  transverse  fractures,  of  that  area  which  has  in  each  case  been  the  seat  of  eruptive 
action.  Prof.  J.  D.  Whitney  and  I  had  occasion  to  observe  another  instance,  more  forcibly  striking  for  its  grandeur  than 
any  of  the  foregoing.  Following  the  crest  of  the  unbroken  range  of  the  Sierra  Nevada,  from  south-southeast  to  north- 
norlhwest,  one  arrives  in  the  northern  part  of  the  County  of  Plumas  at  a  sudden  change  of  scenery  and  rocks.  The  meta- 
morphic rocks  of  the  Sierra  Nevada  are  broken  off  and  terminate  in  a  Ijne  apparently  at  about  right  angles  with  the  direc- 
tion of  the  crest  of  the  former.  The  rugged  and  wild  scenery  which  they  present,  particularly  in  this  county,  gives  way 
abruptly  to  one  equally  remarkable  for  its  beauty  and  geological  interest.  The  high  volcano,  Lassen's  Peak,  rises  in  the  dis- 
tance, amidst  an  intensely  volcanic  group  of  hills.  Low  forest-clad  ridges  built  of  lava,  and  separated  by  meandering  plains 
covered  by  meadows,  stretch  southward  from  it,  gradually  sloping  down  towards  the  line  of  dislocation,  and  there  abut 
against  the  wall  of  metamorphic  rocks.  The  whole  surface,  though  lower  than  the  summit  range,  is  entirely  volcanic.  Sev- 
enty miles  northwest  from  Lassen's  Peak  rises  Mount  Shasta,  more  sublime  even  than  the  other.  Between  both  is  a  deep 
depression,  through  which  the  Pit  River  takes  its  course  from  northeast  to  southwest,  at  little  altitude  above  the  sea. 
This  depression  is  situated  at  the  very  place  where  the  crest  of  the  Sierra  Nevada  should  pass,  and  is  directed  transversely 
to  it.  Northwest  of  Mount  Shasta,  Prof.  W.  II.  Brewer  found  the  continuation  of  the  Sierra  Nevada,  with  similar  char- 

(119) 


82 


RICHTHOFEN NATURAL   SYSTEM 


These  appear  to  be  the  most  essential  geographical  and  geological  relations 
regarding  the  mode  of  distribution  and  occurrence  of  volcanic  rocks.  It  is  perfectly 
evident  that  no  one  of  them  singly  was  the  chief  cause  of  their  occurrence,  nor  were 
all  of  them  together  ;  but  each  of  them  had  a  marked  influence,  either  upon  the  direc- 
tion and  the  location  of  the  orifices  of  the  fractures,  or  upon  the  mode  of  ejection. 
Keeping  in  view  these  different  relations,  I  shall  dwell,  in  the  rest  of  this  chapter,  more 
particularly  on  the  connection  between  the  occurrence  of  volcanic  rocks  and  the  con- 
figuration of  the  surface.  This  subject  may  appear  to  be  beyond  the  scope  of  this 
paper.  But  the  examination  of  every  question  which  relates  to  the  inner  connection 
between  the  phenomena  attending  the  ejection  of  volcanic  rocks  will  aid  in  disclosing 
the  true  nature  of  these,  and  promote  the  knowledge  of  the  principles  of  their  natural 
system.  We  will  endeavor  to  answer  the  following  questions :  Was  the  particular 
structure  of  certain  portions  of  the  crust  of  the  globe,  which  is  indicated  by  the  situa- 
tion of  elevated  regions  on  its  surface,  among  the  causes  of  the  eruptions  of  volcanic 
rocks  ?  or  were  the  inequalities  of  level  on  the  surface,  in  the  volcanic  regions,  due  to 
the  processes  attending  and  the  agencies  causing  the  eruptions? 

The  answer  to  both  questions  must  be  in  the  affirmative.  The  peculiar  structure 
of  the  earth's  crust,  at  those  places  where  mountain  ranges  and  highlands  rise  on  its 
surface,  appears  to  have  influenced  in  a  great  measure  the  distribution  of  the  volcanic 
rocks,  because  those  among  them  which  are  accompanied  by  the  latter,  had  been 
elevated  before  the  time  of  the  eruptions,  and  the  adjoining  lowlands  were  not  the 
theater  of  eruptive  activity.  But,  on  the  other  hand,  it  is  evident  that  the  ejection  of 
volcanic  rocks,  or  rather  those  subterranean  processes  of  which  they  were  one  of  the 

acter  in  rocks  and  scenery  to  that  which  that  range  has  in  other  parts.  It  appears  that  a  gap  of  more  than  one  hundred 
miles  in  length  lias  been  formed  in  the  region  of  the'two  volcanoes,  by  the  displacement  of  a  portion  of  the  Sierra  Nevada, 
which  was  probably  bounded  by  two  lines  of  fracture  transverse  to  the  direction  of  the  mountain  range,  and  has  subsided 
thousands  of  feet,  and  that  then  an  immense  accnmulation  of  volcanic  rocks  filled  up  the  gap,  and  closed  in  building  up  the 
two  giant  volcanoes.  Other  lines  of  dislocation  which  have  given  vent  to  volcanic  rocks,  and  which  have  more  frequently 
been  noticed,  are  directed  parallel  to  mountain  ranges.  Of  such  nature  appears  to  be  the  abrupt  descent  of  the  Sierra 
Nevada  towards  the  Great  Basin,  which  has  been  the  theater  of  violent  eruptive  action  ;  and  probably  the  relations  on  the 
western  slope  of  the  Rocky  Mountains  are  of  a  similar  nature.  The  Vihorlat-Gutin  Range,  in  Hungary,  offers  a  striking 
illustration  of  an  extensive  accumulation  of  volcanic  rocks  along  the  foot  of  a  preexisting  mountain  range,  though  the  dis- 
location is  not  conspicuous  in  that  country,  on  account  of  the  deposition  of  recent  sediments  which  filled  up  the  Hungarian 
Basin.  There  may  be  some  affinity  between  these  modes  of  occurrence  of  volcanic  rocks  and  the  manner  in  which  they  are 
met  with  in  certain  areas  of  flat  or  hilly  countries,  surrounded  by  ranges  composed  of  ancient  rocks.  The  best  illustration 
is  afforded  by  the  Basin  of  Transylvania.  The  undulating  country  of  the  interior  is  encircled  by  high  ranges  consisting  of 
ancient  formations,  which  are  lined  on  their  inner  side  with  volcanic  rocks.  Hungary  itself  affords  a  similar  instance,  though 
less  regularity  is  perceptible  ;  and  the  same  structure  is  somewhat  approached  in  the  geological  relations  of  Bohemia.  We 
may  also  mention,  as  recalling  that  mode  of  occurrence,  certain  depressions  between  the  two  summit  ranges  of  the  Sierra 
Nevada,  such  as  the  Basins  of  Sierra  \ralley  and  Lake  Tahoe,  which  are  encircled,  first  by  a  ring  of  volcanic  rocks,  and 
then  only  by  the  metamorphic  and  granitic  rocks  which  form  the  bulk  of  the  Sierra  Nevada.  Or  the  "  Parks  "  of  the 
Rocky  Mountains.  Their  geology,  it  is  true,  is  almost  unknown.  An  interesting  description  of  the  San  Luis  Park,  the 
greatest  among  them,  recently  published  (see  American  Journal  of  Science  and  Arts,  November,  1867),  shows  that  the 
elevated  rim  of  its  basin,  which  is  estimated  at  eighteen  thousand  square  miles,  is  made  up  of  ancient  formations,  and  that 
volcanic  rocks  encircle  more  immediately  the  extensive  plain  forming  its  bottom,  which  is  itself  composed  of  volcanic 
sediments,  thns  completing  a  structure  that  reminds  of  that  of  Transylvania,  in  more  than  one  respect.  If  we  look  for 
instances  on  a  grander  scale,  we  may  find  some  analogy  with  the  mode  of  occurrence  of  volcanic  rocks  as  just  described, 
in  the  volcanic  ranges  encircling  the  Basin  of  the  Pacific  Ocean.  And  it  may  not  be  out  of  place  if  we  call  attention  to  the 
similarity  with  these  circular  basins  which  is  presented  by  the  configuration  of  the  surface  of  the  moon. 

(120) 


OF    VOLCANIC    ROCKS.  83 

resulting  events,  exerted  a  powerful  reaction  upon  the  further  promotion  and  accelera- 
tion of  those  motions  of  the  crust  by  which  the  irregularities  of  level  of  the  surface  were 
produced.  This  may  be  inferred,  in  the  first  place,  from  the  fact,  that  in  and  along 
volcanic  belts,  elevation  and  plication  of  sediments  have  taken  place,  probably  in  every 
instance  ;  both  modes  of  disturbance  have  affected,  partly  sediments  anterior  in  age 
to  the  eruptive  action,  and  partly  such  as  were  contemporaneous  with  and  posterior  to 
it ;  and  there  may  be  frequently  noticed  a  diminution  of  the  intensity  of  these  dis- 
turbances, in  an  inverse  ratio  with  the  distance  from  the  volcanic  belt.29  But  besides 
this  fact,  observation  has  placed  it  beyond  doubt,  that  in  the  second  half  of  the  Ter- 
tiary period,  and  after  it,  a  greater  aggregate  amount  of  relative,  and  probably  too  of 
absolute  elevation  has  taken  place  generally  over  the  globe  than  had  been  the  case  for 
ages  before.  The  altitude  of  the  rocks  composing  the  summits  of  the  highest  moun- 
tain ranges  and  table-lands  of  the  present  time,  exceeds  that  which  they  had  in  the 


29  This  evident  connection,  in  volcanic  regions,  of  elevation  and  plication,  as  cause  and  effect,  appears  to  be  one  of 
the  gravest  objections  against  the  general  validity  of  the  theory  of  Mr.  Hall  on  the  mode  of  formation  of  mountain  ranges, 
though  the  incongruity  of  an  explanation  of  the  granitic  wedges  by  metamorphism  in  situ,  with  the  chemical  nature  of 
granitic  rocks,  the  total  insufficiency  of  a  gradual  sedimentary  deposition  for  accounting  for  a  foot-for-foot  subsidence  of 
large  areas,  and  the  difficulty  of  finding  any  natural  cause  of  the  assumed  fact  that  the  belts  of  greatest  amplitude  within 
each  area  of  subsidence  should  have  been  elevated  to  mountain  ranges  in  preference  to  other  neighboring  parts,  are  objec- 
tions of  scarcely  less  weight.  It  is  a  well-established  fact,  that  the  Alps  existed  as  a  mountain  range,  at  least  from  the  time 
of  the  Jurassic  period  (though  probably  long  before  it),  and  have  gained  in  height  in  later  periods.  Yet  the  Jurassic  and 
Cretaceous  strata  are  as  much  contorted  as  are  those  of  more  ancient  age,  and  the  same  is  true  in  a  still  greater  measure 
of  the  Tertiary  Flysch.  If,  therefore,  Mr.  Vose,  in  his  Orographic  Geology  (p.  68),  arrives  at  the  conclusion  :  "  So  far  as 
we  have  examined  the  facts,  no  other  hypothesis  than  that  of  the  slow  sinking  of  vast  masses  of  yielding  sediments  can  at 
all  satisfactorily  account  for  the  plication  and  other  evident  effects  of  a  compressive  force  so  invariably  exhibited  in  moun- 
tain districts,"  the  more  recent  formations  of  the  Alps  confirm  this  view  as  little  as  the  above-mentioned  example  regarding 
the  volcanic  belts.  It  may  be  mentioned,  in  this  connection,  that,  while  Mr.  Hall's  theory  would  necessarily  imply  an 
increase  in  the  intensity  of  plication  towards  the  axis  of  greatest  accumulation,  which  is  considered  by  him  to  have  been 
coincident,  after  the  elevation,  with  the  axis  of  the  mountain  ranges,  this  conclusion  is  not  confirmed  by  the  structure  of 
the  Alps.  In  Vorarlberg  and  northern  Tyrol,  which  offer  probably,  of  all  parts  of  the  Alps,  the  most  normal  and  regular 
structure,  all  strata,  commencing  with  those  of  Triassic  age,  are  least  disturbed  where  they  are  nearest  to  the  axis,  and  the 
plication  increases  towards  the  foot  of  the  range.  Superposition,  by  contortion,  of  older  formations  on  those  of  more  recent 
origin,  is  encountered  with  increasing  frequency  of  instances  and  growing  distinctness,  in  crossing  the  parallel  waves  from 
those  nearest  to  the  axis,  towards  the  northern  foot  of  the  range.  Another  objection,  nearly  related  to  the  foregoing,  is 
this,  that,  granting  that  a  trough-like  depression  would  be  able  to  cause  plication  by  the  settling  of  the  yielding  strata 
towards  the  axis  of  greatest  subsidence,  the  waves  thereby  formed  would  have  their  steeper  inclination  towards  that  axis, 
and  their  flatter  slopes  directed  outwardly,  since  a  greater  resistance  would  oppose  the  motion  of  the  lower  strata  than 
would  be  offered  to  the  higher  ones.  The  analogous  instance,  to  which  reference  has  been  made,  of  a  wave  rolling  towards 
the  beach,  shows  plainly  the  effect  of  the  retardation  suffered  by  the  lower  portion  of  the  water  by  friction,  in  the  steepening 
of  that  side  nearest  to  the  place  towards  which  motion  is  directed,  and  the  final  throwing  over  of  the  top  in  the  same  direc- 
tion. A  familiar  instance,  which  repeats  more  exactly,  though  on  a  small  scale,  the  conditions  suggested  by  Mr.  Hall's 
theory,  is  afforded  when  a  viscous  mass,  such  as  resin  or  pitch,  moves  downward  on  a  slightly  inclined  plane.  If  the  neces- 
sary conditions  are  given  for  the  formation  of  waves,  their  steeper  sides  will  always  be  on  the  lower  end,  or,  on  that  which 
would  be  nearest  to  the  axis  of  a  trough-like  depression.  It  is  obvious  that  this  shape  is  the  reverse  of  that  of  the  waves 
of  plication  which  Mr.  Rogers  and  others  have  observed,  and  on  which  even  Mr.  Hall's  theory  is  partially  founded.  This 
theory,  which  was  first  proposed  for  a  mountain  range  in  which  recent  formations  do  not  occur,  and  where,  therefore, 
not  all  of  these  objections  could  be  raised,  may,  if  somewhat  modified,  have  its  limited  applicability  for  partially  explaining 
the  mode  of  formation  of  certain  mountain  ranges ;  but  numerous  arguments  appear  to  preclude  its  general  applicability, 
especially  so  in  the  case  of  the  European  Alps,  and  probably  not  less  in  the  case  of  the  Himalaya.  It  appears,  too,  to  be 
quite  inadmissible  in  the  case  of  the  Rocky  Mountains  ;  and  as  regards  the  Sierra  Nevada,  only  a  chain  of  the  most  arbi- 
trary assumptions  would  be  able  to  give  it  an  apparent  validity  for  the  explanation  of  any  features  in  the  structure  of  that 
range. 

(121) 


84  RICHTHOFEN  —  NATURAL   SYSTEM 

Cretaceous  period  by  thousands  of  feet.  This  is  not  only  true  for  those  ranges  which, 
like  the  Andes  and  Rocky  Mountains,  have  been  intensely  volcanic,  but  also  for  such 
as  show  a  less  immediate  connection  with  eruptive  activity  in  the  volcanic  era,  as  is 
the  case  with  the  Pyrenees,  the  Alps  and  the  Himalaya.  The  elevation  which  suc- 
ceeded the  Cretaceous  period  appears  to  have  everywhere  been  slow,  intermittent,  and 
partly  retrograde,  in  the  Eocene  epoch,  while  its  chief  phase  is  traceable  to  the  Miocene 
and  subsequent  epochs,  by  the  relative  altitude  above  the  level  of  the  sea  to  which 
sediments  of  different  ages  have  been  raised. 

There  can  scarcely  be  any  doubt  that  this  acceleration  of  the  changes  of  level, 
and  the  extrusion  of  volcanic  rocks,  have  had  an  intimate  connection.  But  the  mode 
of  this  connection  has  by  no  means  been  ascertained,  and  appears  to  be  quite  an  intri- 
cate subject.  It  was  formerly  believed  that  the  eruptive  rocks  were  themselves  the 
upheaving  agents,  their  intrusion  and  ejection  having  been  the  mechanical  means  of 
the  elevation  of  mountain  ranges.  But  geological  observation  and  argumentation  have 
conclusively  demonstrated  that  such  could  not  be  the  case,  and  made  it  probable  that 
both  elevation  and  eruption  are  the  most  conspicuous  symptoms  of  different  agencies 
which  were  dependent  on  one  common  cause.  When  that  first  theory  had  to  be  aban- 
doned, another  was  put  in  its  place,  to  the  effect  that  the  ejection  of  rocks  must  have 
been  attended  by  subsidence  ;  and.  although  we  shall  show  in  the  following  pages 
that  the  rise  of  the  crust  greatly  predominated  in  the  vicinity  of  the  eruptions,  there 
can  be  no  doubt  that  subsidence  was  probably  in  all  cases  among  their  intricate  effects. 
But  the  cause  generally  ascribed  to  it  is  not  at  all  capable  of  explaining  it.  The 
assumption  that  eruption  must  be  attended  by  subsidence  is  usually  made  on  the  ground, 
that  the  removal  of  a  certain  volume  of  matter  by  ejection,  from  a  deep-seated  place, 
would  cause  the  formation  of  a  vacuity,  corresponding  in  extent  to  the  volume 
removed,  and  that  consequently  the  overlying  portions  of  the  crust  would  settle  down. 
This  assumption  appeared  to  be  corroborated  by  the  observation  that  the  region  sur- 
rounding a  volcano  subsides  during  its  activity,  while  it  rises  in  periods  of  rest.  But 
it  is  evident  that  the  size  of  a  current  of  lava  is  not  proportionate  to  the  settling  of  an 
area  of  hundreds  of  square  miles  to  the  amount  of  several  feet.  If,  moreover,  the 
views  here  advocated,  regarding  the  cause  and  mode  of  eruptive  activity,  are  correct, 
that  is,  if  the  ejection  of  rocky  matter  is  the  effect  of  the  increase  of  volume  which  it 
undergoes  on  passing  into  the  state  of  aqueous  fusion,  then  the  eruption  is  only  the 
discharge  of  the  surplus  matter  which  has  no  room  within  the  space  of  the  fissure  ;  no 
vacuity  could  therefore  be  formed,  and  the  commonly  adopted  cause  of  the  subsidence 
would  not  exist.  There  are,  however,  two  other  causes  which  would  produce  sub- 
sidence, and  have  probably  been  acting  in  every  case  of  massive  and  volcanic  erup- 
tions. The  first  of  them  is  the  contraction  of  the  liquid  mass  in  the  conduits,  by  cool- 
ing. Its  amount  must  be  considerable  in  proportion  to  the  space  of  the  fissure,  but 
small  when  compared  with  the  volume  of  a  mountain  range,  on  the  surface.  Its  effects 
will  be  local  and  abrupt.  In  the  case  of  volcanoes,  it  is  the  probable  cause  of  the 
familiar  phenomenon  of  the  subsidence  of  the  bottom  of  craters,  and  of  those  less  fre- 
quent cases  where  whole  portions  of  the  cone  are  suddenly  engulfed.  As  regards 
extensive  accumulations  of  rocks  by  massive  eruptions,  there  are  certain  features  of 
(122) 


OF   VOLCANIC    KOCKS.  85 

the  configuration  of  their  surface  which  cannot  be  more  plausibly  explained  in  any 
other  way.  To  these  belong  the  sudden  breaks  in  the  continuity  of  their  surface, 
which  consist  sometimes  in  elongated  and  steep  walls,  thousands  of  feet  in  height,  and 
miles  in  length,  or  in  crateriform  or  semi-circular  basins,  and  in  other  more  or  less 
abrupt  depressions,  which  may  be  chiefly  noticed  where  large  regions  are  uniformly 
covered  with  granite,  porphyry  or  volcanic  rocks.  This  cause  will  affect  the  mass  of 
the  eruptive  rock  itself,  but  not  perceptibly  the  surrounding  country.  Yet,  this  is 
subsiding  when  a  volcano  is  active,  and  it  can  be  definitely  proved  in  the  case  of  the 
andesitic  ranges  of  Hungary,  that  during  the  epochs  of  the  massive  eruptions,  the  gen- 
eral rise  has  been  repeatedly  interrupted  by  the  subsidence  of  the  suroundings  of  the 
theater  of  activity.  To  these  changes  applies  probably  the  second  of  the  causes  alluded 
to.  Whenever  a  fissure  is  filled  by  liquid  matter  injected  from  below,  its  surroundings 
must  necessarily  become  heated,  and,  by  their  expansion,  produce  a  slight  increase  of 
the  rise  of  the  surface.  This  heat  escapes,  in  the  case  of  a  volcano,  chiefly  in  the  epochs 
of  its  activity,  by  the  emission  of  lava,  vapors  and  boiling  water,  and  other  phenom- 
ena associated  with  volcanic  action.  Massive  eruptions  will  have  been  attended  by  the 
escape  of  heat  on  a  much  larger  scale.  They  appear  to  have  been  often  accompanied 
by  an  extremely  violent  emission  of  hot  water,  as  may  be  inferred  from  the  great 
accumulation  of  deposits  of  silica  in  some  volcanic  countries,  or  from  the  immense 
overflows  of  extensive  regions  by  volcanic  mud  :  this  occurs  in  Hungary  and  on  the 
western  slope  of  the  Sierra  Nevada  on  so  grand  a  scale  as  to  almost  exclude  the  possi- 
bility of  its  having  originated  merely  from  volcanic  action,  especially  as  no  volcano  is 
visible  from  which  they  could  have  escaped.  These  processes  must  of  course  have  had 
the  effect  of  lowering  the  temperature  of  the  masses  surrounding  the  fissure  to  some 
distance  from  it,  and  of  producing  subsidence  during  the  eras  of  activity.  Yet  they 
are  not  sufficient  to  explain  the  extent  to  which  it  has  often  taken  place,  though  it  had 
in  no  case,  in  the  vicinity  of  the  theaters  of  eruptive  action,  more  effect  than  to  reduce 
locally  the  amount  of  elevation. 

A  few  examples  will  suffice  to  show  how  vast  are  the  changes  of  level  which 
have  taken  place  since  the  commencement  of  the  volcanic  era,  and  to  demonstrate  their 
connection  with  the  other  manifestations  of  vulcanism  during  the  same  era.  An  in- 
structive instance  is  furnished  by  the  country  situated  between  the  Pacific  coast  and 
the  Rocky  Mountains.  The  labors  of  several  distinguished  geologists  have  made  us 
acquainted  with  some  of  the  main  features  of  its  complicated  structure  ;  but  it  is  only 
by  the  detailed  examination  of  some  of  the  most  important  portions,  together  with  the 
accurate  determination  of  the  age  of  several  of  the  sedimentary  and  metamorphic  forma- 
tions, made  by  and  under  the  direction  of  Professor  Whitney,  that  the  foundation  has 
been  laid  for  an  exact  exploration  of  the  entire  western  part  of  North  America,  which  is 
now  proceeding  with  rapid  steps,  and  promises  to  give  important  contributions  towards 
the  solution  of  the  questions  .discussed  in  this  essay.  There  appears  to  have  existed,  as 
we  mentioned  before,  an  ancient  granitic  era  in  that  region.  But  it  is  as  yet  impos- 
sible to  recognize  the  relation  of  this  ancient  granite  and  the  metamorphic  rocks  by 
which  it  is  accompanied  to  the  ancient  or  to  the  present  configuration  of  the  surface. 
In  the  western  countries,  those  formations  are  concealed  by  the  immense  overlying 

P  (123) 


00  RICHTHOFEN  —  NATURAL    SYSTEM 

accumulation  of  Palteozoic  and  Mesozoic  sediments,  and  the  subsequent  denudation  of 
these,  though  grand  in  the  extreme,  has  only  been  sufficient  to  expose  to  view  the 
ancient  granite  in  some  localities,  among  which  we  mentioned  that  on  the  Colorado 
River.  Other  masses  of  ancient  granite  are  visible  at  the  surface  in  numerous  places  in 
the  Great  Basin,  but  their  exact  relations  still  remain  to  be  determined,  while  in  the 
Rocky  Mountains  they  are  one  of  the  prominent  features,  partaking  largely  in  their 
structure. 

The  next  period  of  interest,  in  regard  to  the  occurrence  of  eruptive  rocks,  is  that 
of  the  deposition  of  Triassic  and  Liassic  sediments  in  great  aggregate  thickness,  which 
were  found  by  Whitney  to  extend  from  the  Pacific  coast  far  into  the  Great  Basin.  They 
prove  that  all  this  country  was  then  still  submerged  beneath  the  sea,  while  the  Rocky 
Mountains  formed  probably  a  broad  belt  elevated  above  it.  It  is  in  this  period  that 
were  ejected  the  quartzose  porphyries  of  the  County  of  Plumas,  in  northern  California, 
almost  contemporaneously  with  those  of  the  southern  Alps.  Whether  this  event,  which 
was  probably  not  limited  to  the  region  mentioned,  was  attended  by  any  changes  in  the 
configuration  of  the  surface,  cannot  yet  be  decided.  After  it,  however,  they  must  have 
taken  place  on  a  grand  scale,  reminding  one  of  the  emergence  of  the  central  body  of  the 
Alps  from  the  sea  after  the  deposition  of  the  iufra-Liassic  limestones.  The  ejection  of 
granite  found,  it  appears,  almost  the  whole  country  embraced  between  the  western 
slope  of  the  Sierra  Nevada  and  the  eastern  slope  of  the  Rocky  Mountains  lifted  out  of 
the  sea.  But  changes  greater  than  those  which  had  preceded  them,  appear  to  have 
attended  and  followed  those  gigantic  outbreaks.  They  were  probably  due,  in  a  great 
measure,  to  the  intense  metamorphism  which  was  connected  with  them,  and  has  com- 
pletely changed  the  petrographical  character  of  the  preceding  sedimentary  deposits. 
When  the  volcanic  era  commenced,  which  was  probably  in  the  Miocene  epoch,  all  the 
ancient  formations  of  the  Sierra  Nevada  were  nearly  turned  upon  their  edges,  and  the 
depressions  of  the  surface  in  the  Great  Basin  were  filled  with  saltwater.  But  the  alti- 
tude of  the  entire  plateau  above  the  level  of  the  sea  was  probably  insignificant  at  that 
time  compared  with  what  it  is  now,  as  may  be  inferred  from  the  fact,  that  rivers  did  then 
flow  on  the  present  western  slope  of  the  Sierra  Nevada,  parallel  to  its  crest,  which  they 
could  not  have  done  if  that  slope  had  had  its  present  inclination.  We  must  imagine 
that  where  the  great  mountain  range  raises  now  its  lofty  summits,  a  hilly  country  ex- 
tended then,  ascending  slightly  to  the  east.  The  first  outbreaks  of  volcanic  rocks  found 
those  rivers  still  flowing  in  their  beds,  as  is  proved  by  the  higher  sediments  in  the  old 
river-channels,  which  consist  of  volcanic  tufas.  But  great  changes  occurred  after  the  first 
commencement  of  the  volcanic  era,  changes  which  contributed  probably  more  towards 
imparting  to  the  western  portion  of  North  America  its  present  features  than  any  which 
had  preceded  them.  The  volcanic  belt  extending  along  the  entire  western  coast  of 
America,  had  its  greatest  breadth  between  the  coast  of  California  and  the  Rocky  Moun- 
tains, and  the  eruptive  activity  has  been  violent  over  that  vast  region.  It  appears  that 
to  that  era  is  due  the  principal  part  of  the  elevation  of  the  high  table-lands.  Such,  at 
least,  is  the  case  in  their  western  portion.  The  crest  of  the  Sierra  Nevada  must  have 
been  elevated  at  a  quicker  rate  than  its  western  foot,  as  distinc  traces  are  left  in  the 
gravel  deposits  that  those  ancient  rivers  which  were  flowing  parallel  to  its  crest  have 
d-.'l, 


OF   VOLCANIC   ROCKS.  87 

been  gradually  turned  from  their  channels,  overflowing  their  banks  at  several  places  in 
succession,  and  taking  a  course  down  the  slope,  until  an  entirely  new  system  of  water- 
courses was  created,  at  right  angles  to  the  crest,  the  steep  ravines  and  gorges  of  which  are 
now  one  of  the  characteristic  features  of  the  Sierra  Nevada.  It  appears  that  the  changes 
of  level  since  the  inauguration  of  the  volcanic  era  have  also  been  progressing  on  a  grand 
scale  in  the  eastern  part  of  the  Great  Basin,  where  the  eruptive  activity  had  probably 
even  more  gigantic  proportions  than  in  the  west.  The  central  portion  of  the  Rocky 
Mountains  was  raised,  according  to  the  estimate  of  Dana,  about  seven  thousand  feet  since 
the  Cretaceous  ;  and  its  eastern  part  from  one  to  two  thousand  feet,  since  the  Miocene, 
according  to  the  observations  of  Hayden. 

These  figures  render  evident  the  great  elevation  which  the  mountain  mass 
between  the  Sacramento  and  the  Missouri  must  have  undergone  since  the  commence- 
ment of  the  volcanic  era.  Twice  in  the  history  of  that  country,  since  the  Triassic 
period,  may  there  be  recognized  an  extraordinary  intensity  of  all  those  changes  which 
we  must  ascribe  to  subterranean  agencies.  The  first  instance  was  in  or  about  the 
Jurassic  epoch,  when  strata,  which  appear  to  have  been  quietly  deposited  during  pre- 
ceding eras,  were  elevated,  and  a  gigantic  intrusion  and  ejection  of  granitic  masses  was 
attended  by  an  intense  and  wide-spread  metamorphism,  by  which  disturbances  and 
plications  were  promoted;  the  second,  in  the  volcanic  era.  It  is  probable  that  similar 
phases  to  those  mentioned  will  be  recognized  throughout  the  entire  range  of  the  Andes, 
the  geological  structure  of  which  appears  to  offer  much  similarity  in  different  parts.  To 
the  events  of  the  volcanic  era,  chiefly,  will  have  to  be  ascribed  the  connection  of  both 
parts  of  the  continent,  though  it  may  have  been  prepared  by  that  preceding  era  of  in- 
tensified actions,  which  manifested  itself  in  the  ejection  of  the  granite  of  the  Sierra 
Nevada,  and  appears  to  have  left  no  less  distinct  traces  in  other  portions  of  the  range 
of  the  Andes.  We  may  still  note  a  peculiar  difference  in  the  mode  of  the  changes  of 
level  if  we  proceed  across  the  continent  from  west  to  east.  It  appears  that  the  narrow 
strip  of  land  adjoining  immediately  the  western  coast  has  been  subjected  rather  to 
periodical  oscillations  than  to  any  lasting  changes  of  level,  while  the  great  elevation  of 
the  mountain  ranges  and  highlands  must  be  chiefly  ascribed  to  the  circumstance  that 
all  changes  have  acted  there  essentially  in  the  same  direction,  producing  the  elevation 
of  extensive  regions.  This  would  explain  why  the  western  descent  of  the  Andes  has 
been  periodically  increasing  in  steepness  and  the  strip  between  them  and  the  coast  is  of 
little  width.  The  latter  appears  to  correspond  to  the  boundary  between  an  area  of 
elevation  to  the  east  and  an  area  of  subsidence  to  the  west,  one  of  which  was  especially 
subjected  to  the  manifestations  of  vulcanism,  while  in  the  other  it  has  left  no  recogniz- 
able signs.  It  is  quite  different  on  the  eastern  side  of  the  range  of  the  Andes,  where, 
in  both  parts  of  the  continent,  a  slow  rise  has  taken  place,  which  has  connected,  during 
the  volcanic  era,  that  mountain  range  with  other  ranges  farther  east,  by  those  exten- 
sive low-lands  which  are  so  important  a  feature  as  regards  the  extraordinary  produc- 
tivity of  both  parts  of  the  continent. 

Similar  to  these  are  the  relations  presented  by  the  European  continent,  in  regard 
to  which  we  will  only  mention  a  few  prominent  facts.  During  the  porphyritic  era  and 
the  time  immediately  succeeding  it,  great  changes  of  level  had  taken  place  on  that 

(125) 


00  RICHTHOFEN  —  NATURAL    SYSTKM 

continent.  It  appears  that  in  Middle  Germany  the  Triassic  period  designates  the  com- 
mencement of  an  era  of  repose,  while  in  the  Alps,  where  the  porphyritic  outbursts  took 
place  in  the  Triassic  age,  the  Liassic  strata  still  participated  in  the  disturbances  and 
elevations,  which  partly  attended  the  porphyritic  era  and  partly  succeeded  it.  There- 
after followed  another  period  distinguished  by  the  comparatively  small  amount  of  dis- 
turbances which  took  place  in  it.  Subsidence  appears  to  have  prevailed  in  its  first  part, 
while  towards  its  end,  during  the  second  half  of  the  Eocene  epoch,  that  renewed  rise 
commenced  which  contributed  so  much  towards  imparting  to  that  mountain  range  its 
present  configuration.  In  surveying  the  general  features  of  the  geology  of  Europe,  it 
will  be  found  that  the  changes  of  the  boundaries  of  the  continent  and  the  sea,  as  pro- 
duced by  rise  and  subsidence,  have,  in  the  period  intermediate  between  the  porphyritic 
and  the  volcanic  era,  been  inconsiderable  in  proportion  to  the  length  of  time;  and, 
though  grand  in  the  totality  of  their  results,  they  are  probably  far  surpassed  by  those 
which  have  been  produced  since  propylite  reopened  the  eruptive  activity.  Xone  of 
the  prominent  mountain  ranges,  it  is  true,  have  been  called  into  existence  since  then. 
Some  of  them,  of  little  importance  and  consisting  completely  of  volcanic  rocks,  have 
been  formed,  and  sedimentary  formations  have  been  folded  quite  extensively,  so  as  to 
form  hilly  regions  ;  but  the  main  ranges  had  existed  before,  and  only  underwent  an  in- 
crease in  volume,  though  one  of  considerable  magnitude,  and  the  rate  of  elevation  appears 
to  have  been  greatest  with  those  mountain  ranges,  during  the  period  indicated,  which 
have  the  greatest  altitude  at  present.  The  amount  of  elevation  which  Eocene  and 
Miocene  strata  have  experienced  in  the  Alps,  Pyrenees,  and  others  of  the  prominent 
mountain  ranges  justifies  this  conclusion,  while  the  mode  of  occurrence  of  these  forma- 
tions in  the  Alps  allows  us  to  infer  that  their  central  portion  was  elevated  at  a  more 
rapid  rate  than  either  the  northerly  or  the  southerly  part.  It  is  worthy  of  note  that 
not  alone  in  the  Alps,  but  quite  generally  in  Europe,  the  Jurassic  and  Cretaceous 
strata  have  undergone  only  little  more  disturbances  than  those  of  Eocene  age.  In  the 
belts  elevated  during  the  volcanic  era,  all  three  formations  participate  ordinarily  at 
nearly  equal  rates  in  the  structure  of  the  low  ranges  of  the  hills  stretching  between 
the  main  ranges  and  girting  their  foot.  The  conclusion  appears  therefore  to  be  justi- 
fied that  in  the  volcanic  belts,  at  least,  the  elevation  of  the  Jurassic  and  Cretaceous 
has  chiefly  taken  place  during  the  volcanic  era. 

There  may  be  distinguished  a  twofold  mode  of  elevation  in  the  volcanic  era  :  it 
manifested  itself  either  in  a  secular  rise  of  continental  areas,  or  in  the  elevation  of 
mountain  ranges,  as  may  be  exemplified  by  referring  to  the  difference  in  the  mode  of 
elevation  between  the  Andes  and  the  countries  adjoining  them  to  the  east. 

The  distinction  is  not  so  definite  if  we  refer  to  the  European  continent.  Among 
the  regions  elevated  during  the  volcanic  era  may  be  chiefly  noticed  a  broad  belt  ex- 
tending from  the  Alps  through  the  Turkish  Peninsula,  Asia  Minor,  Armenia,  and 
Persia  to  the  Himalaya,  and  continuing  less  distinct  westward  to  the  Pyrenees.  The 
three  main  ranges  have  experienced  the  greatest  amount  of  elevation  within  this  belt. 
The  rest  of  it,  which  is  distinguished  by  the  folding  of  the  Nummulitic  strata  over  its 
entire  area,  marks  the  region  of  the  next  greatest  intensity  of  the  elevating  forces  within 
a  much  more  extensive  area  of  continental  rise.  The  increase  of  altitude  has  been  com- 
(126) 


OF   VOLCANIC    ROCKS.  89 

paratively  inconsiderable  over  this  area  ;  yet  it  is,  in  its  totality,  of  greater  import  for 
the  configuration  of  the  surface  of  the  planet  than  the  production  of  the  mountainous 
and  hilly  regions  mentioned.  For  it  had  the  effect  of  connecting  regions  which  were 
before  separated  by  the  sea,  and  of  increasing  thereby  the  area  of  continents  in  cer- 
tain directions.  It  is  well  known  that  the  sea,  which  in  the  beginning  of  the  volcanic 
era  extended  in  the  valley  of  the  Danube  to  above  Vienna,  bathed  both  slopes  of  the 
Carpathians,  and  reached  eastward  to  the  foot  of  the  highlands  of  Central  Asia,  has 
since  then  considerably  retired,  and  that  the  secular  rise  of  northern  Africa,  Arabia, 
and  Asia  Minor  has,  also  in  these  countries,  enlarged  the  continental  areas. 

So  much  time  has  elapsed,  in  most  countries,  since  the  main  phases  of  the  vol- 
canic era,  that  the  influence  of  the  events  connected  with  them  upon  elevation  and 
subsidence  has  probably  relaxed.  It  appears  that  portions  of  the  areas  of  elevation 
have  suffered  a  periodical  subsidence,  not  only  since  that  time,  but  also  during  the 
volcanic  era.  But  its  aggregate  amount  having  been  in  most  of  them  less  than  that  of 
elevation,  the  final  effect  manifests  itself  generally  as  a  rise  ;  though  it  appears  that 
in  recent  times  some  regions,  chiefly  those  which  are  situated  on  the  northern  and 
southern  boundaries  of  the  great  area  of  continental  elevation,  are  undergoing  a  more 
marked  subsidence.  Even  now,  however,  those  countries  chiefly  are  rising  which 
were  distinguished  by  eruptive  activity  in  the  volcanic  era. 

• 

These  few  instances  will  suffice  to  show  how  great  have  been  the  changes  in  the 
configuration  of  some  portions  of  the  surface  of  the  globe  during  the  relatively  short 
geological  time  which  has  elapsed  since  the  commencement  of  the  volcanic  era,  and  how 
much  superior  they  appear  to  have  been  to  those  which  had  been  going  on  during 
preceding  periods  of  much  longer  duration.  Those  facts  which  are  known  in  relation 
to  this  subject,  allow  to  infer,  that  the  changes  of  level  were  proceeding  at  an  acceler- 
ated rate,  during,  and  probably  also  in  the  time  next  preceding,  the  volcanic  era,  and 
that  the  elevation  was  limited  to  certain  belts  of  great  extent  which  were,  in  the 
majority  of  cases,  distinguished  from  neighboring  regions  by  eruptive  activity.  But 
this  is  not  true  for  all  cases,  since  there  are  some  regions  the  elevation  of  which  was 
especially  grand,  and  in  which  very  few  rocks,  or  none  at  all,  were  ejected.  In  ex- 
amining into  the  causes  of  the  connection  of  elevation  with  the  other  events  of  the 
volcanic  era,  we  must  therefore  keep  in  view  the  distinction  of  those  cases  where  it  was 
attended,  and  those  where  it  was  not  attended,  by  eruptive  activity. 

As  regards  the  first  cases,  we  may  refer  for  their  explanation  to  our  previous 
theoretical  considerations  of  the  causes  and  effects  of  the  formation  of  fractures.  If 
the  increase  of  volume  by  the  slow  and  perfect  crystallization  of  viscous  masses  be- 
low the  crust  was  the  cause  of  the  rending  of  fissures,  a  slow  rise  must  have  preceded 
this  process.  That  this  was  so  on  the  European  continent,  is  verified  by  the  fact,  that 
the  Eocene  strata  had  been  elevated  quite  considerably  when  the  first  of  those  sedi- 
ments, which  can  be  proved  to  have  been  contemporaneous  with  eruptive  activity, 
were  deposited.  If  then  the  relief  from  pressure  caused  the  extensive  crystallization, 
at  the  depth  to  which  the  fissures  reached,  of  masses  which  had  been  held  before  in  a 
viscous  condition  by  the  pressure  itself,  an  accelerated  rise  would  be  the  effect ;  and  if 

(127) 


90  RICIITHOFEN NATURAL   SYSTEM 

the  repeated  rupturing  of  the  crust  caused  a  repetition  of  this  pfocess,  in  the  way  in- 
dicated before,  epochs  of  an  accelerated  rise  would  alternate  with  those  of  a  retarded 
elevation.  This  theory  will  explain  why  a  very  considerable  rise,  and  of  greater  ex- 
tent than  that  which  had  preceded,  attended  the  phenomena  of  the  basaltic  epoch. 

Although  we  must  consider  this  subterranean  process  as  the  chief  cause  of  con- 
tinental elevations,  so  far  as  they  attended  the  phenomena  of  eruptive  activity,  it  was 
certainly  not  their  only  cause.  There  can  be  no  doubt  that  the  process  of  metamor- 
phism  must  have  promoted  those  changes.  As  far  as  we  understand  the  conditions 
required  for  its  production,  it  appears  that  they  could  not  be  created  by  any  other 
known  processes  in  a  degree  similar  to  that  in  which  we  must  necessarily  suppose  them 
to  have  attended  the  ejection  of  rocks,  whether  its  mode  be  that  of  massive  eruption 
or  of  volcanic  activity.  Even  on  the  surface  there  are  no  places  where  we  can  observe, 
at  the  present  time,  metamorphic  processes  of  greater  intensity  than  near  the  orifices 
of  volcanoes,  or  in  solfataras,  or  at  the  theaters  of  other  processes  attending  or  succeed- 
ing volcanic  activity  ;  nor  is  there  at  any  other  places  more  evidence  afforded  of  subter- 
ranean metamorphic  processes.  The  latter  will  exceed  those  visible  on  the  surface  in 
nearly  equal  proportion  with  the  increase  of  temperature  and  pressure  in  depth  ; 
while  we  may  conclude  theoretically,  that  the  neighborhood  of  fissures  filled  with 
heated  substances  from  below  would  not  only  have  a  higher  temperature  than 
other*  portions  of  the  crust,  but  also  allow  of  a  comparatively  free  circulation  of  water, 
on  account  of  its  shattered  condition.  In  respect  to  metamorphic  action,  as  in  all 
other  respects,  the  grandeur  of  the  phenomena  of  massive  eruptions  would  make  us 
presuppose  that  they  would  have  given  rise  to  a  far  more  extensive  and  intense  meta- 
morphism  than  the  insignificant  processes  of  volcanic  activity.  The  occurrence  of 
ancient  eruptive  rocks  in  belts  of  highly  metamorphosed  sediments  removes  it  almost 
beyond  doubt  that  they  are  connected  in  the  relation  of  cause  and  effect.  It  is  assumed 
that  metamorphic  processes,  on  account  of  the  entering  of  water  into  the  composition 
of  rocks,  and  their  crystallization,  must  be  attended  by  an  increase  of  volume,  and 
will  have  the  mechanical  effect  of  elevating  the  surface. 

We  may  thus  obtain  a  few  hints  as  to  the  causes  of  the  changes  of  level  which 
those  portions  of  the  surface  of  the  globe  which  were  the  theater  of  eruptive  activity 
in  the  volcanic  era,  have  undergone  since  its  inauguration.  The  agencies  to  which 
they  were  due  must  be  even  more  intricate  than  the  motions  of  the  crust,  because  the 
movement  in  one  direction  may  be  the  resultant  of  several  forces  working  in  opposite 
directions.  Increase  of  volume  by  crystallization  below  the  crust,  in  those  regions 
where  the  formation  of  fractures  relieved  the  masses  from  pressure,  is  probably 
the  most  potent  of  these  agencies.  Metamorphism  would  work  in  the  same  direction, 
while  loss  of  heat  would  cause  subsidence.  All  these  agencies  have  their  first  and 
common  cause  in  the  formation  of  fractures  by  an  upward  tension,  and  all  other 
phenomena  of  vulcanism  are  the  immediate  or  mediate  effects  of  that  same  cause, 
which  itself  results  from  the  gradual  cooling  of  the  globe. 

There  remain  those  cases  to  be  considered  where  some  of  the  most  prominent 
mountain  ranges,  in  the  structure  of  which  volcanic  rocks  take  no  part,  were  elevated 
during  the  volcanic  era,  at  a  much  greater  ratio  even  than  those  in  which  these  rocks 
(128) 


OF   VOLCANIC    ROCKS.  91 

occur.  The  Alps  and  the  Himalaya  are  the  most  striking  instances  of  this  kind.  The 
first  of  these,  and  probably,  too,  the  other,  had,  as  we  mentioned  before,  undergone  a 
considerable  elevation  in  and  after  the  porphyritic  era,  but  have  probably  changed  their 
altitude  very  little  in  the  next  following  periods.  The  acceleration  of  their  elevation 
during  the  volcanic  era  appears,  however,  to  have  greatly  exceeded  that  of  the  Andes. 
The  coincidence  in  time  of  these  events,  and  of  all  the  phenomena  characteristic  of  the 
volcanic  era,  justifies  the  supposition  that  they  are  connected  in  their  origin. 

If  we  contemplate,  in  its  relations  to  the  volcanic  era,  the  great  elevated  belt  of 
which  the  Alps  and  Himalaya  form  the  axis,  we  find  it  to  consist  of  three  parallel 
zones.  The  central  one  comprises  those  two  mountain  ranges  and  the  broad  moun- 
tainous country  which  connects  them,  and  which  owes  its  configuration  chiefly  to  the 
events  of  the  volcanic  era.  The  Alps  and  Himalaya  are  free  from  volcanic  rocks.  This 
is  also  true  as  regards  the  central  portions  of  those  chains  which  branch  off  from  the 
Alps  in  a  southeasterly  direction,  and  of  those  (as  far  as  their  geology  has  been  ex- 
plored) which  extend  westerly  from  the  Himalaya.  The  farther  one  proceeds  in  the 
Turkish  peninsula  towards  the  east,  the  more  frequent  are  the  monuments  of  eruptive 
activity  of  the  volcanic  era;  they  are  known  in  Servia  and  Bulgaria,  in  the  north,  and 
in  Macedonia,  Thracia  and  Bpirus,  in  the  south.  They  increase  in  similar  ratio  as 
one  proceeds  westerly  from  the  Himalaya,  through  the  highlands  of  Armenia  to  Asia 
Minor.  If  it  is  considered  that  active  volcanoes  are  particularly  numerous  in  those 
regions  where  the  elongated  portions  of  two  different  continents  have,  as  it  were,  a 
tendency  to  connection,  the  analogy  with  the  mode  of  distribution  of  volcanic  rocks 
in  the  regions  mentioned  is  conspicuous.  The  system  of  the  Alps  with  its  southeastern, 
and  that  of  the  Himalaya  with  its  western  branches,  were  disconnected  before  the  Ter- 
tiary period,  and  were  united  into  one  mountainous  belt  during  the  same.  The  eruptive 
activity  contemporaneous  with  this  slow  process  was  remote  from  the  main  axis  where 
this  had  existed  as  an  elevated  range  before,  and  approached  it  more  and  more,  from 
both  sides,  increasing,  at  the  same  time,  in  intensity,  until  it  culminated  in  that 
region  where  the  connection  of  both  systems  was  effected.  Massive  eruptions  and 
volcanic  activity  have  ceased  completely  in  this  central  zone. 

North  of  this,  is  another  zone,  which  was  distinguished  in  its  entire  length  by 
the  intensity  of  eruptive  action  in  the  volcanic  era.  It  stretches  from  Central  Asia  by 
the  Caspian  Sea,  the  Caucasus,  the  Crimea,  the  Carpathians  and,  in  branches,  through 
central  Germany  to  central  France.  In  the  European  portion  of  this  zone,  there  are 
only  recognizable  in  thermal  and  mineral  springs  the  last  feeble  remnants  of  former 
volcanic  activity,  while  they  are  somewhat  more  energetic  in  that  portion  which  is 
situated  on  the  Asiatic  continent.  A  third  zone,  not  less  distinguished  for  the  in- 
tensity of  eruptive  action  in  the  volcanic  era,  accompanies  the  main  axis  to  the  south. 
It  traverses  India,  and,  in  the  Bay  of  Bengal,  is  connected  with  the  volcanic  belts  of  the 
Indian  Archipelago,  while  it  continues  to  the  west  through  Arabia,  Syria,  Palestine,  to 
the  Mediterranean,  where  it  comprises  the  active  volcanoes  of  the  Grecian  Archipelago 
and  Italy,  and  is  connected  by  Sardinia  and  southern  Spain  with  the  Azores.  Volcanic 
activity  still  continues  in  this  belt,  but  the  period  of  the  massive  eruptions  has  passed 
long  ago. 

(129) 


92  RICHTHOFEX  —  NATURAL   SYSTEM 

The  entire  belt,  composed  of  these  three  zones,  has  to  be  considered  as  one  grout 
area  of  elevation,  characterized,  at  the  same  time,  over  its  greater  portion,  by  the  rem- 
nants of  the  eruptive  activity  of  the  volcanic  era.  No  portion  of  it,  however,  has.  as 
we  mentioned  before,  experienced  an  amount  of  additional  elevation  during  that  period 
equal  to  that  of  those  mountain  ranges  which  existed  before  it,  and  among  which  the 
Alps  and  Himalaya  are  the  most  prominent.  In  these  cases  there  cannot  have  been  any 
connection  between  the  elevation  and  the  ejection  of  the  rocks  to  the  surface,  because 
these  do  not  occur.  But  this  does  not  preclude  the  connection  between  the  elevation 
and  those  agencies  which  are  the  causes  of  eruption.  It  has  been  often  demon- 
strated that  the  changes  of  level  must  have  been  accompanied  by  the  formation  of 
such  fractures  as  would  be  closed  next  to  the  surface,  and,  though  allowing  of  an  in- 
trusion of  plastic  matter  from  below,  would  prevent  its  extrusion  to  the  surface.  It  is 
probable  that  fissures  of  this  kind  may  have  been  chiefly  formed  where  high  mountain 
chains  are  composed  of  metamorphic  rocks,  since  they  frequently  exhibit  vertical  disloca- 
tions, partly  parallel  to  their  axis,  and  partly  at  right  angles  to  it,  by  which  the  strata 
on  one  side  have  been  moved  thousands  of  feet  above  the  other,  and  which,  notwith- 
standing, did  not  give  vent  to  the  ejection  of  rocks.  These  faulting  fissures  appear  to 
form  frequently  the  lateral  boundaries  of  mountain  ranges  and  the  limits  of  the  mani- 
festation of  vulcanism,  and  both  these  circumstances  allow  us  to  infer  that  they  extend 
downward  to  great  depth.  Though  one  of  the  main  features  in  the  geological 
structure  of  the  Alps,  they  have  hitherto  been  little  examined.  But  it  is  probable  that 
their  formation  was  coincident  in  time  with  the  main  phase  of  elevation,  that  is, 
with  the  volcanic  era.  If,  then,  in  that  part  of  the  crust  over  which  the  Alps  are  ele- 
vated, such  fractures  were  formed  as  did  not  open  on  the  surface,  then  all  the  agen- 
cies below  would  cooperate  towards  elevation  alone.  The  expansive  force  produced 
by  aqueous  fusion  would  find  no  vent  for  the  discharge  of  the  masses  by  which  the 
volume  had  increased,  nor  would  there  be  any  opportunity  given  for  the  escape  of 
heat  by  hot  water  and  other  means  ;  while,  on  the  other  hand,  the  masses  in  depth 
would  be  relieved  from  pressure  by  the  formation  of  this  kind  of  fractures  in  the  same 
way  as  by  those  which  would  allow  the  passage  of  liquid  matter  to  the  surface.  In  the 
case  of  the  latter,  an  immense  amount  of  force  is  spent  in  other  modes  of  action,  while 
in  the  first  case  it  could  be  applied  almost  exclusively  to  elevation.  Besides  this  direct 
action,  however,  we  must  also  keep  in  view,  that  in  those  regions  where  fissures  would 
not  be  open  at  the  surface,  the  conditions  required  for  metamorphic  action  would  be 
given  on  a  particularly  grand  scale.  Gases  and  water  would  not  reach  the  surface,  but 
be  employed  in  depth  in  promoting  metamorphic  action  over  vast  regions,  and  increas- 
ing the  rate  of  elevation.  It  may  be  by  processes  of  this  kind  that  already  in  ancient 
times,  contemporaneously  with  an  accelerated  elevation,  were  formed  those  granitic 
wedges  surrounded  by  broad  belts  of  foliated  metamorphic  rocks,  which  are  peculiar 
to  certain  mountain  ranges,  particularly  to  those  which  also  in  the  volcanic  era  have  not 
been  the  theater  of  eruptive  activity.  The  intrusion,  from  below,  of  heated  masses  into 
fissures  closed  towards  the  surface,  would  give  all  the  conditions  required  for  the  exhibi- 
tion, on  a  grand  scale,  of  those  processes  of  hydropyric  metamorphism  which  DaubnV 
has  rendered  probable  by  his  experiments  and  theoretical  deductions. 
(130) 


OF   VOLCANIC    ROCKS.  93 

Leaving  aside  these  theoretical  considerations,  and  comparing  only  the  differ- 
ent facts  mentioned  in  regard  to  the  changes  of  level  which  have  taken  place  on  the 
surface  of  the  globe  since  the  inauguration  of  the  volcanic  era,  we  are  forced  to  the 
conclusion  that  they  must  be  intimately  connected,  as  regards  their  causes,  with  the 
other  phenomena  by  which  this  era  was  distinguished  from  preceding  periods.  Eleva- 
tion and  eruptive  activity,  even  when  locally  not  quite  coincident,  are  coordinate  effects 
of  the  cooling  of  the  globe  ;  but  while  the  one  is  its  immediate  effect,  the  other  results 
from  it  only  by  the  concurrence  of  other  agencies,  which  by  themselves  alone  would 
have  been  incapable  of  producing  results  of  such  magnitude.  As  there  are  distinct 
phases  in  the  history  of  eruptive  action,  dependent  on  and  marking  the  evolution  of 
the  globe,  so  we  may  recognize  different  stages  in  the  mode  of  manifestation  of  the 
elevating  forces.  In  those  elevations  which  took  place  during  the  volcanic  era,  there 
are  certain  peculiar  features  evident,  among  which  may  be  mentioned  :  the  increase  in 
altitude  of  those  mountain  ranges  included  in  the  volcanic  belts  ;  the  union  into  one 
main  chain  of  several  smaller  ridges,  the  axes  of  which  are  situated  on  a  line  ;  the 
lateral  connection  of  parallel  ranges  into  highlands  ;  the  oblique  connection  of  the 
ends  of  main  chains,  the  axes  of  which  are  situated  on  parallel  lines,  but  are  remote 
from  each  other  longitudinally,  by  broad  belts  of  mountainous  regions  (of  this 
character  is  the  connection  of  the  Alps  and  the  Himalaya)  ;  the  elevation,  final- 
ly, above  the  level  of  the  sea,  of  large  areas  which  had  been  submerged  before, 
whereby  distant  mountain  ranges  were  connected  by  extensive  lowlands,  the  size 
of  the  continents  increased,  and  their  outlines  rendered  more  uniform.  The  mode 
of  these  great  changes  of  the  configuration  of  the  surface  of  the  globe  "was  probably 
a  repetition  on  a  large  scale  of  similar  changes  which  had  been  going  on  in  former 
eras  of  eruptive  activity.  It  is  not  our  object  here  to  develop  them.  But  throughout 
the  course  of  these  changes  there  appears  to  be  conspicuous  a  tendency  to  connect,  in 
certain  directions,  what  was  before  disconnected,  to  increase  in  size  and  render  more 
definite  certain  areas  of  elevation,  and  to  separate  them  from  other  areas  of  subsidence, 
which  appear  to  have  been  likewise  increasing  in  extent.  We  may  even  recognize  an 
increase  of  this  tendency  during  the  volcanic  era,  as  the  phenomena  connected  with 
the  basaltic  epoch  have  been  far  more  widely  and  generally  distributed  over  the  areas  of 
volcanic  belts  than  those  of  the  preceding  (propylitic  and  andesitic)  epochs.  There  must 
be  certain  general  laws  which  regulate  the  directions  in  which  the  connection  of  discon- 
nected parts  takes  place.  Their  knowledge  is  still  very  limited.  The  admirable  manner 
in  which  Dana  has  laid  out  the  grand  outlines  of  the  arrangement  of  the  islands  of  the  Pa- 
cific, has  indicated  the  way  by  which  we  may  look  for  the  solution  of  this  problem.  If  the 
structure  of  the  crystallized  crust  of  the  globe  gives  the  most  probable  cause  of  the 
definite  directions  which  may  be  recognized  in  the  outlines  of  continents  and  chains  of 
islands,  it  should  be  borne  in  mind,  that  the  direction  of  structural  planes,  if  they 
exist,  must  vary  gradually  in  depth,  in  the  same  ratio  with  the  chemical  composition, 
and  that  they  may  at  the  depth  of  andesitic  compounds  be  different  from  what  they 
would  be  in  the  granitic  depths.  The  singular  way  in  which  the  connections  by  ele- 
vation are  effected,  and  which  has  been  already  partly  pointed  out  by  Dana,  appears  to 

Q  (131) 


9-1  RICUTHOFEN —  NATURAL   SYSTEM    OF   VOLCANIC    UOCKS. 

find  only  an  explanation  when  such  changes  of  the  direction    of  the  structural  planes 
are  assumed  to  take  place  in  depth.  •   u~ 

The  tendency  to  simplify  beyond  their  present  conditions  the  general  outlines 
of  the  configuration  of  the  surface  of  the  globe,  continues  probably  without  intermis- 
sion, though  an  era  of  comparative  repose  has  followed  the  violent  exertions  of  vulcun- 
ism  of  the  volcanic  era.  The  great  backbone  of  the  Eastern  continent,  comprising  the 
Alps  and  the  Himalaya,  and  the  second  great  belt  which  comprises  the  Andes,  and, 
more  properly,  encircles  the  whole  Pacific  Ocean,  are  at  the  present  epoch  the  main 
features  in  the  orography  of  the  globe.  Both  have  derived  their  prominent  position 
from  the  events  of  the  volcanic  era.  The  conclusions  at  which  we  have  arrived  in 
these  pages  have  been  drawn  chiefly  from  observations  made  in  regions  of  prominent 
interest  in  either  belt,  and  it  is  for  this  reason  especially  that  we  believe  that  at  least 
some  of  them  will  be  found  to-be  susceptible  of  general  application. 


I  am  fully  aware  how  imperfect  is  this  attempt  to  develop,  in  the  correlations  of 
the  volcanic  rocks,  the  principles  of  their  natural  system.  This  system  should  be,  as 
we  remarked  before,  not  a  classification  of  objects  by  certain  conspicuous  properties, 
but  a  classification  of  objects  by  their  definite  mutual  relations.  We  have  not  to  make 
the  divisions,  but  to  find  them.  We  shall  be  able  to  do  this  with  greater  or  less  per- 
fection, in  the  same  measure  as  those  relations  are  studied  and  firmly  established. 
Much  is  needed  to  arrive  at  this  end.  It  can  only  be  achieved  after  evidence  has  been 
accumulated  by  the  combined  and  harmonious  labors,  in  different  countries,  of  geolo- 
gists in  the  field  as  well  as  in  the  laboratory.  The  range  over  which  both  modes  of 
observation  extend  from  year  to  year,  and  the  field  on  which  their  results  meet,  is  in- 
creasing in  a  surprising  degree.  Yet  eruptive  rocks  form  ordinarily  an  object  of  less 
exact  observation  than  those  of  sedimentary  origin ;  and  it  cannot  be  denied  that  har- 
monious observation,  by  which  alone  can  be  established  the  principles  of  comparative 
petrology,  is  rendered  nearly  impossible,  as  long  as  no  uniform  system  of  nomenclature 
is  applied.  These  pages  are  the  result  of  an  attempt  to  add  one  more  to  the  many 
contributions  towards  establishing  the  foundation  of  both,  and  to  show  the  value  which 
the  study  of  eruptive  rocks  and  of  those  mutual  relations  which  comprise  their  natural 
system  promises  to  have  for  approaching  the  solution  of  some  of  the  highest  problems 
of  geological  science. 


(132) 


TABLE  OF  CONTENTS. 


PACK. 

INTRODUCTORY 1 

TUB  NATURAL  SYSTEM  OF  VOLCANIC   ROCKS 5 

ORDER  FIRST  :  RHYOLITE 12 

ORDER  SECOND  :   TRACHYTE 17 

ORDER  THIRD  :    PROPYLITE 20 

ORDER  FOURTH  :   ANDESITE 25 

ORDER  FIFTH  :   BASALT 26 

CORRELATION  OF  THE  FIVE  ORDERS  OF  VOLCANIC  ROCKS 28 

Laws  Relative  to  the  Age  of  Massive  Eruptions 29 

Laws  regarding  the  Mutual  Relations  of  Massive  Eruptions  and  Volcanic  Activity  32 

RELATION  OF  VOLCANIC  ROCKS  TO  ANCIENT  ERUPTIVE  ROCKS 35 

Correlation  of  Age  and  Texture 39 

Correlation  of  Age  and  Composition 41 

Correlation  of  Eruptive  Rocks  in  regard  to  their  Geographical  Distribution 43 

ON  THE  ORIGIN  OF  VOLCANIC  ROCKS 46 

1 .  Origin  of  Massive  Eruptions 47 

2.  Origin  of  Volcanic  Action 60 

3.  Other  Theories  regarding  the  Origin  of  Volcanic  Rocks 68 

RELATION  OF  THE  DISTRIBUTION  OF  VOLCANIC  ROCKS  TO  THE  CONFIGURATION  OF  THE  SURFACE 

OF  THE  GLOBE  .  . .  78 


(133) 


THIS   BOOK  IS  DUE  ON  THE  LAST  DATE 
STAMPED   BELOW 


RENEWED  BOOKS  ARE  SUBJECT  TO  IMMEDIATE 
RECALL 


JUN  6     1968 
DSREC'O 


LIBRARY,  UNIVERSITY  OF  CALIFORNIA,  DAVIS 

Book  Slip-50m-12,'64(F772s4)458 


371921 

Richthofen,  F.P.W. 

The  natural  system 
of  volcanic  rocks. 


QE461 
R52 


PHYSICAL 
SCIENCES 
LIBRARY 


LIBRARY 

UNIVERSITY  OF   CALIFORNIA 
DAVIS 


Call  Number: 


371921  QE461 

RichthofenTT.P.W. 

The  natural  system 
of  volcanic  rocks . 


